I was also thinking of a catch all forum for the network of sites. The cost of vBulletin is a non-issue. We are currently paying $150/month for Vanilla.
There are a couple of issues with moving to a catch all forum with the biggest being migrating content. I hope Vanilla will give us a dump of the data and that it's something that I can parse and upload into a new system.
The other being, I don't know what vBulletin is up to, the have VB5 as well as VB4. I hate the mobile support in VB4 but it looks like they are down for mobile in VB5. People seem to hate VB5 online though, so I don't know if that would be the best option.
There is another forum suite that looks like it would be nice with a nice mobile design called Invision Power Boards (IPB). Bethesda and other use them and it looks pretty similar to VB. Take a look here and let me know what you think: http://www.invisionpower.com/features/