I don't bother too much with Vilani supremacists. I have enough fun with Solomani supremacists and everybody's prejudice against Vargr. But they exist, I guess.
On general Solomani/Vilani conflicts: a setting that has never really been explored is the Regency of Antiama. This, for those vague on the history, was the period when Emperor Zhakirov married Antiama, a Vilani, and then died, leaving a four year old kid as Empress.
GT: Rim of Fire states that the Imperium was ruled by a Regency Council while the Empress was growing up. Antiama was on it, but she had to share power with others.
Who these others were isn't specified, but one of the first things the Empress (Margaret, IIRC) does when she assumes full authority is to create the Solomani Autonomous Region, that is, effectively split the Imperium in two.
Clearly, something had been going on...
The really cool thing about all this, is that Zhakirov's promotion of Vilani to positions of power had occurred at the expense of Arbellatra's old Solomani supporters. Arbellatra is often viewed as the great Imperial heroine, and it's interesting to consider that the Solomani movement more or less originated among her clique.
Of course, Milieu 0 had a kind of a precursor Solomani movement in the early Imperium, but the "present day" one seems to have originated in response to the ouster of the "Arbellatrans" from power.
There is a slight problem, though, in the relationship between "Solomani" nobles and "Solomani" space. The former can be found throughout the Imperium, not just in "Solomani" territory. The de facto partition of the Imperium could only relieve one of the sources of the pressure on the Imperium, but I suppose it was enough to postpone Civil War for a couple of centuries, and thus successful.
Alan B