Advanced Paid Forum Membership Questions

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You can do it with user permissions and the paid subscriptions built into vBulletin.

Setup your supporter usergroups. A, B, C, D, etc.
Setup your forums and their permissions, A, B, C, D

Then after they are a member of usergroup A for X days(soon as they pay, they'll have access to forum A), they are automatically promoted to B, and given access to forum B

Then setup another usergroup promotion, after being in B for X days they are upgraded to C and given access to forum C

And just repeat for however long you want that to go for.