They want to evolve independent from the Rest of Humanity - and it seems like they are not left alone to do so. So to keep the Safety of being left alone and not invaded as soon as the Opportunity is there - they became militant and fight for their Territory Demands.
BUUUUT: One can also guess pure Evil out of the VS's Reason to fight the New Conglomerate and the Terran Republic. Because if they do, they also get a n~ice Chance to TEST their Technology out, they have. While they are actually forced to the Fight the T.R. sooner or later, they shouldn't have any real Reason to fight the N.C. Only the T.R. of those Two would try to force them to their Will sometimes soon.
Sure, the reason of the Sovereignity indeed is acceptable. But just as you wrote, their exact goal is unclear. They are on one hand trying to evolve, but on the other hand, they might just be "religious" puppets trying to oppress everyone with a "free" mind.
It really is interesting that every faction seems to be equally "good" and "evil" at the same time, with the TR being oppressive peacekeepers, the NC violent liberators without any sense of mercy and the VS advanced technocrats possibly controlled by an ancient alien race.
Also, the VS indeed were defending themselves as the Conglomerate never trusted the Vanu's "brainwash", deciding that they had to fight them as well.