First, let me apologize for the length of this post and the following. I tend to wax lengthy (a trait my players hate), but I find I can't help myself.
I'm breaking this into three posts to make it easier for people to parse through, primarily because the first two posts can be used together, or not, without any real modification to what I'm suggesting (you can, of course, modify them to hell and back, but they still can be taken separately). The first two posts are also mostly fully fleshed out ideas. The third post, builds off of both my first two posts, so any modifications to the first two may require modifications to the third - if you want to use it - and it is more of a musing than a fully formed idea.
That aside, start by saying that I've really enjoyed reading through everything you've put together and I fully intend to incorporate it into my own game should it move in that direction (or if I can push it in that direction). Ok, now to the meat of my first post.
I'm a lover of symmetry, so I'd suggest having a fifth civilization running around out there that's diametrically opposed to the Ven. That is, they possess none of the Ven's Virtues and all of the second group of Virtues that you've suggested. This would mean that there's one civilization with all of group 1 (the Ven), one civilization with all of group 2 (described below) and three that split the difference in various ways.You don't need them to show up in play, necessarily, unless you wanted an absolute foil to the Ven.
There are plenty of potential assumed antediluvian civilizations to choose from that are supposed to be contemporaries of Atlantis and Mu. My suggestion: pre-destruction of Atlantis, the Mediterranean was supposed to have been a massive fertile valley, and the Nile River is presumed to have traveled along the entirety of this basin. The Osirians are presumed to have lived in this valley along the Nile. Moreover, it is believed that they built massive earthquake proof megalithic structures, that there was an abundance of conveniences for its citizens (including electricity), and that they were prolific traders with other antediluvian civilizations. This valley could be protected by natural or artificial features which prevented its ruin during the war among the Sorcerer Kings. Or, maybe, the Sorcerer Kings avoided destroying this valley for reasons of their own. Perhaps, as an overt call to the Osiris myth, the key to their resurrection lay within the valley and none of them wanted to risk harming it.
Regardless, ultimately there was a cataclysmic change in the Atlantic (perhaps the destruction of Atlantis) which caused the Mediterranean Basin to flood and this civilization to be destroyed. It is supposed that its peoples moved to higher grounds and resettled in modern day Egypt.
The name "Osirian" can be loosely translated as either "Strength of the Eye" or "He Who Sees the Throne." Assuming that the Sorcerer Kings were most active in Central Asia, this could be a reference to how close the Osirians were to that location (e.g. they could 'see' the throne of the Sorcerer Kings), but I'm not sure I like that interpretation. Rather, I like the idea of that name stemming from before the fall of the Sorcerer Kings. To wit, they were the Sorcerer Kings favored servants and, literally, could 'see the throne' from their place in society. While the Ven were the footmen, foot soldiers, and sacrificial offerings to the Sorcerer Kings; perhaps the Osirians were the court clerks, concubines and research assistants. Still slaves, but ones with chains of precious metal and silk, rather than the course hemp and rough iron that the Ven labored under.
Maybe, if you want to go further than this. and establish them as more of a foil to the Ven, they're not even human (so the peoples of Egypt - after the coming cataclysm - are their cultural, but not physical descendants). Perhaps they're Ven as well. Bred and created by the Sorcerer Kings, but separated into a higher class for unknown reasons. Or, maybe they're the Ven 2.0. Perhaps the Ven were a first experiment in creating useful servant life that - while not a true failure, was neither a true success. The Osirians, then, were the refinement of that ritual and accorded a higher place in society to suit their drastically enhanced usefulness.
Or, the historical record might support the thought that not all of the Sorcerer Kings were power hungry demi-gods intent on forcing their will upon the world. Maybe there was a subset of this ancient race that sought enlightenment and knowledge without abusing their power. Perhaps the war was between these two factions or, perhaps, the war was between the power-mad Sorcerer Kings and this faction merely found itself trapped in the middle. If that were true, then perhaps the Ven embody the worst aspects of the Sorcerer King continuum of potentiality while the Osirians embody the best aspects of that continuum. The other civilizations, somewhat further removed from the Sorceror Kings than the Ven and Osirians split the difference. But, of course, the thought of "good" Sorcerer Kings is just silly. Right? Of course it is ... right?
So, we have the Osirians (Tradition & History), capable of tracing their lineage (human or otherwise) back to the height of the Sorcerer King empire. Their purpose is not to understand the world, or to conquer it, or to hasten the return of the Sorcerer Kings. No, each of those things presumes to overstep the limitations handed down to them by their Masters, and who are they to question the dictates of their immortal creators. Rather, they prepare for the return of the Sorcerer Kings no matter how long that may take. To that end, they construct megalithic buildings that can withstand the test of time, and everything the natural world may unleash. They trade with their neighbors in order to gather information on the civilizations that will one day be subjugated once again. They gather and safely store Artifacts so that they will one day be ready for use by their rightful owners. They also keep to the old ways, and stagnate under the weight of Ages and rules whose purpose may have long ago been forgotten.
Their individual House equivalents could be based around the traditional tasks performed in the Courts of the Sorcerer Kings. For example, the 6 Orders (replaces Houses):
The Order of the Craftsmen (Ingenuity) - Once assistants in the creation of Artifacts, Puzzle Houses, and wonders great and small, these Osirians now focus on gathering the creations of the Sorceror Kings and storing them safely to ensure that they are ready for the return of their Masters. They also keep the minor wonders allowed to the Osirians by ancient laws functional and in working order. In conjunction with this, they police society to ensure that none stray beyond the limits set on their knowledge by edicts handed down by the Sorceror Kings.
The Order of the Viziers (Insight) - Once researchers and scribes, these Osirians now seek to catalog the partially fallen world in order to ensure that their Masters will have the knowledge necessary to reconquer the world. They also keep track of those experiments begun by the Sorcerer Kings that have not yet concluded. But their purpose is not to know these forbidden things, merely to record them, and so they have mastered the art of intentional ignorance: recording their wisdom and, at once, forgetting it. They also police the Osirians to ensure that none plumb the depths of forbidden wisdom.
The Order of the Guardians (Domination) - Once lieutenants and captains, these Osirians now protect the once and future homes of their Masters. They also, when requested by another Order, venture into other lands, and even - occasionally - into the magic tainted battlegrounds of the Sorcerer King's great war, to recover some relic or knowledge that should be taken into the charge of the Osirians. [NOTE: This group basically remains unchanged (although potentially far more effective) if you combine it with the proposed virtue "Unity" discussed in my next post.]
The Order of the Courtesans (Honor) - Once the physical playthings of the Sorcerer Kings, these Osirians now serve as the political and mercantile elite of society. They ensure that all of the aspects of Osirian society move with the same purpose. They also conduct trade missions with other nations, both to ensure the economic security of the Osirians, but also to recover the knowledge and artifacts needed by the other Orders.
The Order of the Priests (Faith) - Once the Ritual assistants of the Sorcerer Kings, these Osirians now lead their nation in fervent and unending rituals dedicated to their Masters. These rituals provide the raw power needed by the Order of the Craftsmen to repair and build artifacts, and help to ensure that the nation remains focused on their rightful tasks and place in history. Members of this Order are forbidden to be literate or receive instruction on the use of Artifacts. This is so they can serve as custodians for the vast storehouses of books and artifacts gathered and preserved by the other Orders without risk of falling prey to temptation.
Patents (replaces Devotion)
The Osirians understand that they are nothing before the might of the Sorcerer Kings, and they know that their place is not to serve a singular member of that great race, but to serve the race in its entirety and at its whim. They internalize the tasks given to their ancestors by the Sorcerer Kings and become better able to serve in specific ways through that process. For example, one Courtesan may seek to serve by internalizing the task of being a delight at parties; while another might be serve by becoming the perfect herald. These are demonstrated by patents awarded, or assigned, by the highest members of the Orders.
Privilege (replaces Style)
The Osirians, through loyal service to the Sorcerer Kings, would be rewarded with unique rights and privileges unknown to other slaves. The ability to go forth into the great cities alone, the ability to marry for love, the ability to own possessions, and etc. While the Ven seek to emulate the Sorcerer Kings with their Style, the Osirians set aside that false benefit in favor of true power. That is, privilege gained through devoted services, unfailing loyalty, and true actions.
Random idea that got stuck in my head on the drive home from work - Each Osirian was given a patent by their Sorcerer King masters which described and defined the limits of their privilege and permissions in society. A contract, inherently one-sided but a contract nonetheless, that put the Osirian and others on notice as to what that individual was permitted to do in society. Some time after the disappearance of the Sorcerer Kings the Osirians gathered in what would become their capital city and enshrined all of the patents that had been handed down through the generations in great books. These patents formed the base of their laws. For example: What weapons may the Order of Guardians wield? An examination of the patents shows that their ancestors were given permission to bear both long swords and spears, but nothing else. For that reason, no proper Guardian will carry a bow or even use a knife against anything more threatening than their dinner.