Is it? I know the population went up a bit not long ago when the class/race restrictions were removed from the game. Haven't touched it in awhile myself. Otherwise you're stuck with Wizardry Online(Permadeath), Planetside 2, Everquest 2(Which people say is worse than Vanguard, but more populated on a few servers), or DCUO(Superhero fun time).
The site is saying the main server is at medium population too, though I don't know how many people that would be.