Jack Vance is one of the major SF writers who never became a marketing superstar. He has been writing for more than half a century, and in mid 1999 he was still around, though poor health has slowed his writing quite a bit.
Vance has a style all his own. If your command of English isn't broad you'll need a thesaurus just to make sense of some of it. He takes long wandering asides (complete with footnotes!) to explain things most writers would ignore, or explain inline in stilted dialog. Some people have problems with this, and say Vance is hard to read. It may be true, but it's worth it. Vance's stories are often set on stages that would be utterly beyond lesser writers, sometimes to where the story line is heavily shadowed by things "out there". A good Vance story is like a multidimensional version of one of those pictures that looks like one thing, and then when you look closely, it looks like something else. His droll sense of humor is threaded through almost everything he writes, too subtle for many casual readers to pick up.
All this sounds bad, but it can't even begin to describe the rush I get when I reread "The Dirdir", "The Servants of the Wankh", or "The Book of Dreams." No other writer has been able to paint so many stories with so wide, yet detailed a brush.
I had planned to write a page for Jack Vance's science fiction stories. However, there are other Vance fans out there who have done better jobs than I could, so I'll point you there:
Two points I'd like to bring up before you hit the Vance webring. First, even some Vance fans seem to have problems interpreting his work. Two common complaints: characterization and societies. Vance's protagonists are all basically simple men with simple motivations. This seems to bother the type of people who believe everyone is a stew of angst and hidden motivations. The fact that the typical Vance adversary is one of the most complex characters you're likely to encounter doesn't seem to matter to these people.
Second, some people think many of Vance's social structures are too far out to be believable. I actually laughed when I read that for the first time. Every single one is based on known cultures in human anthropology, just portrayed in a new setting. These people think Jack's stretching the bounds of suspension-of-belief, and he's just holding up a mirror to Anthro 304. Just another example of his twisted sense of humor...