vBulletin paid subscription
 

Hey there,

I was wondering if there was a mod for vBulletin, that allowed regular members to upgrade to a Plus Member of something (a new group), by paying a monthly subscription via paypal??

And for this they would be able to access the restricted forum categories etc?

This is built into vBulletin. Look for Paid Subscriptions in the AdminCP.
For them to access special forums, you'll need to create a new usergroup and set the permissions for the usergroup to the forums you want them to access.

Nice, i will check that out. But how do users upgrade their account? Like when new users are registering, can they choose free account or paid account?

You can select the group when making the subscription so that members will automatically change to once they donate.