After MUCH time with Microsoft AND Plextor tech support, here's the result:

Short version: The PX-716UF needs a firmware update for Vista. I don't know if this means all PX-716 models or just the UF, but definitely the UF; probably all of them. Plextor is working on a firmware update for the PX-716... no driver update should be necessary, just the firmware update.

Long version: Did much testing with Microsoft and with Plextor. The Vista CD/DVD driver works fine with the PX-716UF for reading/writing single layer dvds. It reads them fine. It just doesn't write them; hangs on writing the header every time. To prove it was not my system, I bought an IDE version drive (I needed another one anyway) - the PX-760A. It works fine for reading/writing for single and dual layer discs under Windows Vista, HOWEVER you must update the firmware to the latest version available on the Plextor site.

Please let me know if you have questions.