I wouldn't have guessed what I did as "my guess". I'm going on the premise that Rebellion itself has specific clues to indicate what it is.
So just saying, "I think it's this" or "I think it's that" isn't deriving anything directly from Rebellion.
Rebellion itself has to correlate with your answer--and do so in a way that couldn't be derived from previous expansions alone.
So--looking specifically into Rebellion...what's there--that was not in previous releases--that backs up your theory(s)?
I don't disagree necessarily with you but I can poke your logic. If confronted with a genuinely "Godlike" being, the very fact that it defied anything conceivable by the Vasari could be enough to drive them insane.
My own idea prior to Rebellion was the nanites--but let's think about it a second. This is "Sins" of a Solar Empire. What's the "sin" in nanites? We could guess hubris, over-dependence, immoral usage or the like but that's just speculation. If it's unleashing the grey dust that will destroy everything then why is there no lore from other races about their colonies vanishing or fleets being lost? We could say, "It hasn't reached their space yet"...but that's back to pure, unsubstantiated speculation...and nothing that requires Rebellion in order to guess.
So what I did notice was how every new Vasari ship we've seen so far looks extremely bug-like. What would be a "sin" would be to capture a species and experiment on it--or even mutate it into your ships--so you could use it's power. What if the whole of Vasari technology is based on using the organic form of a sentient insect species? So their ships--particularly the new Titans and capitals--might actually be lobotomized sentient insects used by the Vasari as ships.
The fear would be that if the insectoids were seeking retribution or were evolving to throw off Vasari incursions (either intelligently or merely instinctively) that not only would they be fighting the Vasari, they'd effectively be fighting the entire basis of their advanced technology--their actions would cause the Vasari would lose everything and be reduced to nothing.
As to the insanity, it may be the that the "dead" insect ships actually weren't and began to work against their supposed masters. or perhaps the witnessing of a world being consumed by billions of living insects was like seeing a vision of the end of the world to the Vasari.
This is all speculation too--but I think the key is the insect-like look of the ships. The only other new and distinctive features that might qualify would be the capability to reduce worlds or some new nanite functions--but the Vasari seem quite comfortable using nanites and have them well controlled.
Lol--I do get it....sadly. Though in my defense, I stopped watching in season 3 as it became obvious. I did tune in for the last of the final season though. I should have known better. I'm hoping Stardock isn't trying that route. They claim they aren't.