Basically, I don't have an answer, but complain to Nero and see if you can get them on it. You might take a look at the NeroHistory logs for the burn attempts in both Vista and XP to see if there is any difference.
I have not tried any real burn attempts in Vista, and especially not any DL discs. My Vista log was different in that it did not show DMA status and the interface used was different from XP. I cannot get InfoTools to run. Vista is probably using different drivers for your drive and motherboard, which might add to such a problem. Perhaps media that worked in one setup will not work in the other.
Vista has more DRM restrictions, which might be part of it. I chose not to upgrade to Vista and only run a Beta.