Best to get you info direct from the
source.
But if you buy a vBulletin 4.x license you will never have to pay for a renewal for as long as you want to use VB 4.x. You will have access to every VB 4.x version, 4.0. 4.1, 4.2, ... all the way to 4.9.9 if they make it.
At some point VB 5.x will be released, that is when you'll have to pay again if you want access to VB 5.x but no one knows when or how much that may be.