It's different for every OS version, and usually requires a registry hack. Google will tell you more.
I would love it if Vegas would allow you to set the priority from within.
I don't "think" it's in the hidden preferences, but I'm open to being corrected.
It makes very little difference to Vegas as it will still use every available CPU cycle for rendering. The big difference is that other programs can run more smoothly while the render is still proceeding in the background.
So that means that you set the other programs to a higher or lower priority in order to allow Vegas to render smoothly. Can you elaborate a bit on what specifically has worked for you in this situation?
Probably the best example is to open two instances of Vegas, set one to rendering, and set it's priority a little lower. Then go edit in the second instance. The first render trundles along without interrupting the edit session.
Starting off the same as Rob, but i'd add, well, anything and everything else too! Set the Vegas render to a low priority and then anything else i want to do on that computer while the render is running will run more smoothly.
Probably the single biggest reason i do this is so that any videos i want to watch while will play smoothly even while Vegas is still chugging along in the background.