Great info about vassals. I just vasslized two countries and I was wondering if I needed to keep them as allies (in case I would need other allies when the current alliance will end)
Concerning the "revolts in a peacefully annexed country doesn't make sense" : I fully disagree. You probably have negociated the annexion with the upper class (nobility, merchants, whatever) and it doesn't mean that the people are pleased by the new rulers (and even probably part of the upper class, the inheritors of the former ruler,etc... are upset too).
Also a "peaceful" annexion doesn't mean that the annexed country had much choice. For instance when France annexed Brittany, the idea was the following (from the king of France to the duchess of Britanny) : "either you marry me or the huge army just across the border will knock at your door" . I believe nobody in Britanny (especially the upper class which lost most of its power and influence...I'm not sure the peasanry really cared) was really satisfied by this "diplo" RM and annexion.