We will try to reproduce this here, but we need to know some more details:
1. What Windows OS are you running, service packs installed, etc.
2. What version of Vegas are you using?
3. What version of QuickTime are you using? (run the QuickTime player, Help | About)
5. What is your source media type and project settings?
5. What are the exact settings you are using in your QuickTime rendering template?