The factors that govern their speech is the exact same factors that govern our speech for all languages all over in the real world. V just tossed in all that language factor crap to make it seemingly impossible to learn vasudan. So in reality in the fs universe, vasudans just have the same speech factors as earth does.
So English is based on the day of the week, the month, and the spacial orientation of the speaker as compared to the Emperor? The closest human approximation I can think of, actually, is Navajo, which has so many different verb forms based on so many circumstances you'll go mad just trying to figure them out. A word eight characters long in its anglicized version, na'ildil, means roughly "You are accustomed to eat plural seperable objects one at a time." (Dear god, that's actually more words then there are characters in the Navajo word.) There was a very good reason why the Marines used native speakers of the langauge in WW2 as essentially human cryptographic machines: the langauge is so complex as to be totally impenetrable to an outsider, even after years of observation and analysis of context. Yes, you can learn it, but you would never manage to sound like a native speaker and it would take you at least a couple decades.
Now throw in the added complication of the langauge having been created by someone with non-human pyschology.
Learning Vasudan doesn't sound so easy anymore, does it?