It doesn't really say anywhere that Vasudans have anything like a caste system in their society. In fact, all implications point against it; Vasudans are already much less numerous than Terrans in the galaxy and such task delegation would prevent them from technologically advancing as rapidly as Terrans did by the FS2 period. It is quite probable that there was some sort of caste-type thing earlier in their history due to the higher emphasis on religion and general "order" (in our own history, segregation by birth has almost always been backed by some sort of religion), but this would have slowly faded away with time, just as it did with ourselves. Here's a clip from the FS bible:
Despite the physical differences between humans and Vasudans, their mentalities evolved very similarly. They have strong family structures, and enjoy many similar social activities that Terrans do.
If their general mentalities are similar, the historical path they would have taken would be similar to our own, so we only need to look at our own history to predict what that of the Vasudans would be like. They happen to have a capable emperor with good intentions this time, but that of course will not always be the case. There is always the possibility that Khonsu's successor is some HOL-type fanatic and could turn the Imperium into a fascist regime; after many years with experience from such an adminstration (the VPE), the people would probably have had enough. The royal family obviously still remained in power (both FS1 and FS2), but since Khonsu II's precursor was probably more malevolent, the people would think twice before handing everything over to the emperor's son, even though he would probably denounce his father's actions to gain publicity.
If the Imperium is indeed a true monarchy, I don't expect it will last beyond a couple of generations, as our own history has shown, before either being toppled over by an ambitious fleet admiral or the lower classes themselves, instituting either a new dictatorship or a republic. (the dictatorships eventually collapse though; at some point the people usually revolt, either starting the chain of events again, or putting together a new type of government) Also, I doubt that the people have a complete disdain for a parliamentary government as the VPE was not a true parliament at all, but rather an oligarchy of sorts, as stated in the tech descriptions. The change of name was instated to further seperate the new government from the old one, bringing about a change for the better despite the name; a previously suffering society would adjust to the change quite easily.
I looked into the subject of Vasudan government quite a bit when I was formulating ideas for my campaign; I still have some of the notes.
