I ever wanted to do this and i found a simple solution today.

Setting Vegas in below normal priority (in Task manager before today... :-)) give you the "poor man background rendering solution" by allowing other's Windows tasks to behave normally when actually rendering a something heavy in one Vegas instance.

I wanted a simple trick to automatically start Vegas this way when i wanted to do so.

Here is the tested/proven solution with Vegas 5...

1.Create a little text file with this single line :
start /BELOWNORMAL vegas50.exe

(or replace "BELOWNORMAL" with "HIGH", "LOW" or anything else...)

2.Save the file with .bat extension

3.Put it in the same folder than the normal Vegas executable.

4.Start Vegas each time with the .bat instead of the .exe... Et voilà!

Hope this is usefull for some of you.