[CarlitoBrigante]

I doubt it depends on vBulletin, probably a coincidence. I did loads of upgrades from SMF and never had an increase in spam, at least nothing noticeable.

Are you using ReCaptcha in your vBulletin? Also, in our clients' websites, we usually setup the usergroups so that upon registration users are in a usergroup with limited permissions (limited PMs, limited posting). After X posts and/or X days, they are promoted to the real "registered" usergroup. This can cut a lot of spammers because, for many of them, it is simply not worth it.

Anyhow, if your forum offer some service that requires your users to be fully active from the instant they register, then the above tip won't work well for you.