there are some videos online that show samples of how it's used to increase speed actually.......i was/am skeptical, hence the question here......any other feedback?

I searched for the videos you mentioned. Cheat Engine can access the process viewer.

You don't need a gamer's app in order to change your process thread priority in Windows -- you can do that more easily in Windows Task Manager.

Changing the thread priority is an old trick going back to the earliest days of Windows computing. Whether it has any effect on CPU rendering speed or throughput in a given instance depends on a lot of things which have already been discussed to death, along with the potential fallout (freezing, crashing, overheating, etc.).

If you'll search the forums, you'll find many instances of people reducing the Vegas process priority to solve problems.

IOW, it's nothing new. If your CPU is already running near max during render, it will have little or no effect. It will also have no effect on how the rendering codec addresses the CPU. If you want to play with it, back up your system, turn off all unneeded processes, set the vegas.exe priority in Task Manager, don't surf the web, and report back.

It will have no effect in Vegas whatsoever.

I just recalled, there is a "Use High Priority Rendering thread" option in Vegas Internal Preferences.

IIRC, changing it had no effect on my render times.

EDIT: I just tried high thread priority with a typical HDV->AVCHD render, and the only effect I could imagine is that it would make doing anything else nearly impossible.