You don't want to make users wait 30 minutes before they can participate. Web users are all about everything being instant. 30 mins doesn't sound like a big deal, but you're talking about a medium where large companies spend tons of money just trying to figure out how to get a user through something like a checkout process in 3 clicks instead of 4. A user will sign up, get the 30 min wait period, be doing something else in 5 seconds, and have completely forgotten about FTR in 20 seconds.

Have you guys tried signing up for a service like Mollom? It's a 3rd party service that you can have your form data sent to, they quickly scan through it with their tools, then send back to your site a success/fail/maybe response. I've installed it on a few sites I handle that have spam issues and it has proven very effective for them. I would assume vBulletin would have a plugin to connect to their API.