I think this can be the most frustrating part of video editing sometimes. I'm having a silly issue just at the moment. I rendered a short video the other day. Using my trusty mpeg2 YouTube template. Instead of the usual quality, it ended up all blocky like I'd chosen a really low bit rate when in fact I've whacked up the bit rate to see what happens.
I'm sure we've all experienced those horrible moments when we look at the result of an eternally long render only to be disappointed with the result. SO who really knows the secrets of this black magic art of rendering, codecs, formates etc...
I have a theory, I think it's the computer nerds who writes the software code taking revenge on the world, blaming us normal people because they can't get laid.
We'll it's a theory.
I know this little rant doesn't help the OP but I just couldn't help it after last nights frustrations.
I'd recommend picking an applicable standard template in Vegas for what the final video will be used for ie DVD PAL for UK DVD etc.
Having said that I just know I'll be back at my computer tonight fiddling about with the settings trying to tweak the best of of it.