Yes, we are coming from different assumptions. I find the idea of "Vilani cultural rigidity" as it is frequently expressed to be utterly implausible. And, in fact, contradictory to the (admittedly limited) materials we have on the Vilani. I think this is a complete, albeit common, misreading of the Vilani and a typical example of how games tend to take relatively minor things and turn them into "hats" because there's a general lack of other details.
I agree.
I think it is implausible that there was a single Vilani culture developed in the 300,000 years they advanced on Vland, any more than there was on Earth with a similar period of development. The Vilani were on Vland as long as Terrans were on Terra. It seems unreasonable to me that there are not multiple subcultures of "Vilani" the way there are of Terrans.
I agree again.
The Vilani invented a wide range of technologies faster than Terran humans did. They expanded across an enormous area, met many races and cultures, and were clearly dynamic and adaptable. The Vilani sourcebook portrays them as consensus seeking, but not as "rigid". Also, they admire the individual who makes the right call in an emergency without waiting.
Nitpick - the terrans actually invented stuff faster, we went from space travel to jump drive in just over a hundred years. It took the Vilani centuries.
Both the Vilani and the Terrans used STL ships to explore and colonise - Terrans sent their ships off on thousand year missions for some reason, while the Vilani colonised nearby world to Vland and built an interstellar society before jump drive.
The LATE PERIOD VILANI GOVERNMENT was highly rigid and collapsed as a result. That no more defines the "Vilani" than the Ramshackle Empire defines Terrans as incompetent vandals.
You mean teh Ziru Sirka, administered as three satrapies with a shadow emperor in oversight - it lasted over a thousand years.
4717 BC - Vilani discover jump drive, a period of exploration and trade begins that establishes a Vilani hegemony
1480 BC - establishment of the Bureaux - peacful trade hegemony continues
912 BC - hidden history event, jump 2 discovered
882 BC - consolidation wars, this is when Vilani culture noticeably shifted
473 AD - end of consolidation wars, Ziru Sirka founded - shadow emperor appointed
2314 Ad - Ziru Sirka under new management
I do think that the situation on colony worlds is very different from the situation of the 100 or so worlds where humans "evolved". The fact that they start from spacefaring technology means they don't have layers and layers of ruins, bizarre theories like "the four humors", and all that. But it doesn't mean they are immutable.
The colony should begin with a subculture of the polity that establishes it.
But it doesn't mean that these colonies were uniform in origin, planning, or even that there was only one group colonizing a given world. It especially does not solve the problem that communications are extremely slow and humans naturally diverge when groups are relatively isolated from each other. There's no effective solution to that enormous communication lag. Human populations maintain cohesion by constant interaction and shared experiences.
The British Empire spread British culture - to the governors compound

Leave the areas under direct British control and the native cuture dominates. When the Empire shut its doors many former colonies maintained British culture in their government, legal and education systems.
Different events, different environments, unique technologies, unique personalities and popular movements will all happen on worlds and it'll be weeks or months before people elsewhere even think about reacting to them. Yes, education if it is maintained, will keep some things in common. But school curricula changes and people popular culture doesn't derive from only education.
And it doesn't take long for these differences to change the nature of the cultural base.
It is important, imho, to keep in mind that the people writing about "The Vilani" are almost always writing about the Vilani Imperial government. And, most often, the version encountered by the expanding Terrans. That's like learning about the lives of Appalachia townfolk by discussing the nature of Washington DC politicians.
The same can be said for every race in Traveller, all Aslan are like this, all Vargr are like this, all K'kree are like this... no, just no.
Throw in the many cultures of the Solomani and other aliens, minor races, etc and several thousand years of post-First Imperium and it's going to be quite significantly different.
Obviously, there's a limit to anyone's creativity and also the value of actually making diverse cultures in terms of gameplay. That's why we have one Vilani culture and one Solomani culture and one Aslan culture, etc. But, imho, no one should actually think that represents the facts on the ground. Because it is unlikely in the extreme that it actually would.
I agree once again.
There's certainly nothing wrong with simplifying culture down to something manageable. Everyone does that for whatever value of manageable they personally have. But the fact that it is a simplification and practically unlimited complexity would actually exist ought to be kept in mind.
Notice how The Traveller Adventure gave us details for many different world cultures, but very little cultural diversity within these.
How many major cultures are there on Earth? How many minor cultures, how many sub-cultures...