Did you watch your rendered project on a TV after having rendered in Vegas? Was it right?
The culprit may be in either your Vegas or DVDA settings but we'd first have to know where it comes from.
No big deal but we have to know. And it depends of your cam too; you shoot with some anamorphic lens or with a "built-in" 16:9 from the cam? In this last situation, you must render with a normal 4:3 (NTSC) setting to get the black bars on top and bottom... and probably go into DVDA with same settings (but I'm not sure since I just got into DVDA for a few days now).
Did you ask on the DVDA forum too? It might get you quicker response to work this out with two bunch of people...
... and maybe re-edit your title because rendering 16:9 in Vegas has been asked so many times on this forum that maybe people don't even bother to click on your post.
Maybe just mention that your problem is a "combo-problem" combining both Vegas and DVDA use... ;-)
I use a Canon XL1s which has an "electronic anamorphic" mode, which is pretty much useless. It just means that I loose roughly 25% of my pixels. After running several tests I've concluded that, unless you have 16:9 CCD's (best) or an anamorphic lens (OK), you are better off shooting in the camera's native 4:3 mode, then chop off the top and bottom in post if you really must have 16:9. At least this way, if you ever change your mind, you still have a master shot at the camera's maximum resolution. If I need both 4:3 and 16:9, I make two prints, one at each aspect ratio. I tried generating an anamorphic (widescreen) print in DVA, but this resulted in a less than full screen (letterboxed) image in both 4:3 and 16:3. Not very pleasing. The only plus side is that it would play correctly on both 4:3 and 16:9 monitors as long as the DVD player was set accordingly.