Understand, your original file is
720x480x32
29.97 fps
Upper field first
MPEG-2
Any render will cause some slight quality loss, even to uncompressed avi. Vegas can combine some files together without recompression, as long as they have no effects applied to them. If you add the mpeg-2 files together on the timeline and render to the same exact mpeg-2 format, you wil see “No recompresion on the preview screen, and it’ll fly through the process. If this works for you, there will be NO quality loss.
DV AVI is a good format, better than your mpeg so if you want to edit with effects, that is a good format to use.