Hi CowTR,
You're correct that NO is a vasodialator. What that means is that it helps your blood vessels to temporarily relax, which in turn allows more blood and oxygen into the muscle. Your muscles need that oxygenated blood flow in order to keep working, so in theory NO may have the effect of allowing you to workout for a longer period of time in addition to giving you that "pumped" feeling. How much that translates to actual gains in strength and size, however, I don't know.
Jeremy