With what you have mentioned then, that could be a Vegas memory leak issue somewhere.
Your best solution then is, after you finish your editing and saving your project, you simply restart Vegas and do your rendering….. Surprisingly that is my workflow everyday, although im not having the issue you described, but i find that’s the best way to have flawless renders in Vegas. You see, during your edits and ram preview etc, etc. memory gets depleted dramatically and if you go directly into rendering without a restart, you know you can get errors….. So try that method, even if you did not have that issue, its the same i would recommend.