Star Trek, Spider-Man, and anime have something in common that Tolkien and Vance lack (and I don't think Tolkien belongs in the group you put it in): they're visual media, and not only that, they're visual media with a lot of distinctive clothing and devices. You can watch Star Wars and actually see the uniforms and the light sabers. Of course you'll see a lot more people dressing up for those than for Vance, or for Tolkien or Twilight.

Also, I'm not convinced that the lack of Jack Vance conventions is caused by the different nature of the fandom as opposed to just the smaller size. It's quite possible that the Jack Vance fans who exist are proportionately as passionate as Harry Potter fans.