What's the correct configuration of ffmpeg?
What i mean to say is that something about your audio/video output format from Vegas needs changing. It's probably nothing major.
What specifically are you trying to do? What is your video project resolution, framerate, format, etc? What is your audio sample rate and bit depth?
Post your most recent logfile from your %localappdata%\VoukoderPro\logs folder. That will likely tell us what the problem is.