Right now, vbulletin is not worth investing your money in.
What you will get is a "alpha" software that is frankly being marketed as gold. There are major issues with css and the code is neither here nor there (in terms of full oop). They are constantly changing the base code itself so its hardly something you want to code for.
IPB is a lot better, stable and has a great developer team(same isn't true for vbulletin right now). More than anything else, some of the features that IPB is introducing like Nexus are pretty awesome.
Its your personal choice in the end. vBulletin still has a great coder community(a bit bigger than IPB) and is still a forum of choice for most big boards. Having said that, you need to give IPB a look and then decide.