I have ran many forums (most failed) and been a admin/mod on a few big ones, if you need any help please let me know.
First thing is check the phpbb forums for the security posts and secure your stuff, it is a long process but you only need to do it once and when you want to upgrade to vb the upgrade is very simple, I did a phpbb > vb upgrade a few months ago and they have really perfected the import script.
Just to add you are going to need to find something different, the only forums I have had be any success were ones that either I purchased with users already of ones I was very lucky to get some great exposure I never would have in the first place