well what your looking for I think is not a feature nor is it of vbulletin to my knowledge in working with vb
because our subscription feature is based off how they do things. Your not looking for any usergroup permissions payment settings at all. Your looking to make it strictly category by category. Your looking for something in the middle it looks like.
vbulletin subscriptions just like our subscriptions works simply by charging a user a specific fee to get jumped to a higher usergroup which has permissions you assign per category. Ours works pretty much the same way. My invision of how subscriptions work is to jump users into usergroups where they dont pay like normal users but thats just me.
Now our payment feature which is separate from subscriptions works by a payment structure per usergroup. These groups have to pay these groups dont. Now to further define areas where users do or do not pay you can create categories that have amounts to pay and those that dont. Thats done by placing a charge or not in a specific category but it does not break down into specific cat permissions of payment per usergroup. Thats a whole extra level of permissions that do not exist in the current model.
Your free to place suggestions in the suggestions forum. I dont think personally I have ever seen someone request such indepth a payment model.