payment processor creation questions

[Willem P]

I'm also interested on any info on creating a payment API.

I want to be able to have people buy subscriptions with vouchers.
My client offers an audio tech support service and wants to be able to give physical vouchers to his real world clients, as well as selling vouchers in real world shops.

I've set up an external system for managing vouchers and I have been able to create a payment API class which validates the voucher...but can't work out how to have my system call back vBulletin and let it know that the voucher was accepted as payment.