A vassal can vote whatever he wants, excepted if the master is nominated for a Diplomatic Victory, in which case he will vote for his master even if he doesn't like him or likes the other better.
You can pretty much demand every resource you want from your vassal, even he has only one, he won't refuse unless he really doesn't like you and feels like freeing himself by declaring war. Same goes for the gold per turn AFAIK. However, you can't ask too many techs or a large amount of gold every time you feel like it, to avoid players abusing the game mechanics and beating the heck out of a techer to milk him of all his techs. While you're at war with them, you can ask techs for peace. Once they have capitulated, you have better chances of cutting a deal or getting a tech for free if they don't hate you, but you're not automatically given everything you'd like.
