Insert some code after succesful payment from Paypal

[hygum]

HI

Where should I insert some custom code that make some changes in the Joomla/virtuemart, just after succesful payment confirmation from Paypal. It should be in the callback url which paypal contacts in the background to update the order on virtuemart, not later when user might click back from paypal.

I have virtuemart 2.0.12f and joomla 2.5.7