Are you talking about DV-avi files? Then your 12Gb file is just 1 hour.
Normally we render the video in Vegas to MPEG2 using the mainconcept codec within Vegas and the audio to AC3. Gives these two files exactly the same name and place them in the same folder.
In DVDA, just import the video file and the audio file will follow automaticly.
You will see that your 1h file will fit nicely on the DVD as MPEG2.
Even the larger 2h file will fit easily when you render with a lower bitrate, but this might affect the quality a bit. A solution could be the use of a double layer DVD for the larger file.
Never use the MPEG1 option, since this is meant for a low quality video CD
Theo