Deactivating PayPal gentles up Session data storage

[Flokey82]

Hey guys. After 5 hours of brainmelt, fail and facepalm I need the wisdom of the crowd for a solution.
Problem:
I developed a payment module for Virtuemart and it works. Yeah, it works with a lot of customers. But now, I've got a customer who just wants to use my payment and so I deactivated every other payment method.
Guess what happened... From this moment on, all data was sent via GET in the url and not stored in the Session anymore (I think it was stored in the session before)
After trial and error I can confirm that deactivating the paypal module causes the data of the checkout process being transmitted via GET.. Which completely gentles up my payment module.
I COULD adapt my payment module to deal with this, but that's def. a bug in Virtuemart and it really shouldn't act that way. Has anyone else ever encountered this problem?
How to solve it? (without adapting MY module to deal with the bugs of other modules)
The point is, if I always patch my module to deal with all kinds of stuff caused by others, my code is getting messy and probably incompatible with later versions.
Suggestions? Thoughts?