If I'm not mistaken, I believe this is related to dual core, quad core, and multiple cpus. I think you want to set the number of render threads to at least the number of cores/cpus available for rendering. Use at least 2 for dual cores and at least 4 for quad cores.

I think the only time you might want the number lower than the number of cores available is if you plan on using you computer for other aplications while Vegas is rendering to free up some processing power for those other applications. Otherwise, set the number to at least the number of cores available.