Now excuse me while I celebrate my tenth post and promotion to member.


That should give you edit functionality, too.

You certainly make good contributions, and I felt that you were deserving of an early promotion

Once you have gotten them to accept becomming you vassal, do you then state the money you want? Or as I believe makes more sence, you ask for the money when you are requesting them to become your vassal. Yes?
You ask for the money in diplomacy, as you're trying to get them to become a vassal.

Can you later change the amount they tribute?
You can, but I'm not sure how much of an impact it will have on relations (they might get annoyed at you asking)

Do you state both the amount and the number of turns they must pay?
Yep, that's how a tribute works ingame

and the number of turns they must remain a vassal?
Nope. A vassal is treated in the same way as an alliance, and will remain until you or another faction causes it to be broken.