Going to need more info than that. First of all, Sony hasn't made Vegas since Version 13, knowing which version you are on would be helpful We also need to know which version of that version (like Vegas 16, version 424 or something like that, you can find out in the help section). We also need to know your project settings, what settings you are using when you render, and your system specs.

For render issues, I find it's always helpful to start diagnosing by upgrading video card drivers to see if that helps, as sometimes they can cause issues.