You are right it was a virtuemart component issue - specificallt the file checkout.thankyou.tpl.php that came with the rocketlauncher 1.5 Mynxx package - the file you have is BLANK!!!!!
I replaced it with the same file from another install I had done and POOF magically it started working.
Just thought you might want to know to go and check that specific package.
Where will I find that file? I'm having the same trouble with my mynxx template. It insists on C.O.D. and will not complete the sale by taking the customer to the PayPal screen.