Capitulated Vassal Declares War on Me

The title pretty much sums it up. During a Congress my vassal (whom had capitulated to me) voted against me and when I decided to not accept the decision of the Congress to flip my city, my vassal canceled its capitulation and declared war on me. Hilarious? Check. Annoying? Check.

haha man, that has to be annoying..that has never occurred on my games

They capitulated, so they obviously view you as an oppressor. The ownership of the city in question was just a flashpoint.

Why is it so hard for people to roleplay?