Having never seen the insides of a vBulletin file, I cannot give a knowledgeable or unbiased answer. As has already been said, there are likely very few developers here who have much experience in both software, although there may be a higher percentage on vBulletin's forums.
In the end, I think that the phpBB code is simple understand and can be easily modified to get what you want, once you get an idea of how the phpBB-defined classes and methods/functions work and what they can do.
I think the best way for you to decide which is easier or more efficient is to dive right in on both. I know vBulletin is quite expensive, but since you are asking about it, I assume that you have the resources to purchase at least a short-term license so that you can get a glimpse of the code. Anyway, look through the file structures and code structure and make a decision on which you would feel more comfortable learning and developing in.
On a related note, the one thing that I dislike about phpBB MOD development is the creation of the MODX file. Granted, there are applications online and offline that can aid you in the process, but it still can be time consuming, especially for MODs that have a large number of edits to the core code needed. This can be for the most part avoided by modularizing any new functionality and only editing where needed to call a method/function, but sometimes it is still necessary to edit a large chunk of the core code.
But like I said, and I realize I'm rambling--I tend to do so quite often--I think it's best to take some time to look at both options and see which you feel most comfortable with. I personally enjoy phpBB, but that may simply be because I have very limited experience in developing customizations for other software.