No need to be a fanatic for a software you use. Both vb and phpbb are nice forum scripts. It's true that vb is more advanced but I always appreciate phpbb guys are giving it away free. Considering fact that it's free, phpbb can be a very strong incentive to many people and I see more phpbb boards then vb boards around when I surf on the internet.
I dont agree its used by smaller boards. I know a lot of very large boards using phpbb.
I also dont agree its performance is bad. On the user side, I didnt experienced any extra load on phpbb. Actually one of the sites (which is very crowded) I'm usually visit switched from vbulletin to phpbb and I can say that their load is decreased visibly and now board access is much more faster for members than before.
I am not telling phpbb is better than vb. If I did, my site would be using phpbb, not vbulletin. But I'm just stating that both software has its own advantages or disadvanteges and every Admin can regard one better for his needs according to his priorities. Both software is quite complex in technical terms and I believe developers of the both programs deserves appreciation.
Anyway I agree with filburt1: Competition is always good.