Generally You will have seperate user groups setup for Subscriptions. When a user buys a subscription it changes their usergroup and expires after the xx days you put.
I'm wondering if there is ANY sort of conflict to also just "Changing" a members usergroup to one of the types the Subscription system uses.
Hopefully makes sense

Basically want to setup one of the Subscription groups to be bought via vbPlaza without it interferring with the vbulletin Subscription System