Its impossible to say especially considering the number of plugins and mods that could be enabled with vBulletin, but I'd think your current account should be able to handle 5-10 concurrent users with a "stock" vBulletin installation.

All the reseller accts have the same available CPU the only place you'd get more is on a semi-dedicated or dedicated server. A VPS will probably be a little slower than a semi-dedicated and I only recommend them for developmental purposes where root access is required.