Basically the idea is that I'd create a new group called the Donor group. They'd have ALMOST all the same benefits of a VIP except for a few things (after all, VIP's are awesome, right?). You would receive the Donor group tag for 1 month after the last donation you made. Example:
You donate 07/01/2012
You now have donor tag until 08/01/2012
You donate again 07/10/2012
Your donor tag now ends on 08/10/2012 instead
After your date ends, your group will go back to being a member. VIP's will not receive a donor group badge because they're VIP's and everyone should know that VIP's already donate to the site as it is.
Result: We made a donor class (a long time ago's)
