There really isn’t a need to change the thread priority of a Rendering to make it higher. That could potentially lock up your computer until the whole rendering is done, which doesn’t sound to good to me. Lower is okay, but higher isn’t very necessary
You can optimize settings, but in terms of additional threads, V-Ray will always use as much as it has available to it. If you’re just using the V-Ray express quality toolbars you’ll have to start getting into the settings themselves in order to speed up your renders.
I use a quad core 64bit machine and I always use Lower thread priority. Having it on Lower allows you to multitask and in my view it doesn’t really speed up the process if you have it higher.