They are all-together different animals.
VGC is color for color and ALMOST name-for-name the same as GW paint. Not the same formula, but similar.
VMC is very low in binder, replicates the historic military colors (plus other things like horses) and great for museum pieces and display models. It can be used for gaming with lots of clear coat to keep it on the model.
RMS (Reaper Master Series) is a separate beast made for gamers as well. Colors come in triads (shadow, mid-tone and highlight) to take the guesswork out.
P3\'s (sorry, I have no experience with them) are a relative newcomer and all reports are positive.