No, it won't give you any advantage. The CPU maxes out and, unlike "Spinal Tap," you can't go to eleven.

I usually go the other way (decreasing priority), because Vegas is not a particularly good Windows citizen, and insists on hogging resources when it is rendering in the background. You can make your computer far more responsive, with very little reduction in rendering time, by going down one notch in priority during rendering (but only if you need to use the computer for something else during the render).