There are some very very good security mods to protect phpbb. If you have the budget to spend, go with vbulletin because it is already fairly safe and it is the most widely recognized and understood forum software in the user's standpoint. I feel that is the best reason to go with vbulletin, not for the security.
But that does not mean phpbb is bad. Just different.
100% agreed, many big forums are using Vbulletin 3.6.x as I have seen in the internet.

It is stable too but the license fee is $160...
