Both were master storytellers best known for their science fiction, although both were much more than that. Vance also was a mystery novelist and won an Edgar in 1961 for his novel The Man in the Cage. Banks was as acclaimed for mainstream novels such as The Wasp Factory as he was for his science fiction.
Both also arguably were underrated. Vance won a stack of the top science fiction and fantasy awards, including Hugo Awards, Nebula Awards and the World Fantasy Award, but he generally was ignored by the literary world at large as a "genre" writer. I was surprised and delighted, however, when the New York Times magazine ran a long, insightful piece in 2009 that included praise for Vance from the likes of Neil Gaiman and Michael Chabon.
While Banks was a major figure in Britain, he was less well-known in the U.S. Although he was arguably the world's best space opera writer — his Culture novels are reliably wonderful — he never won a Hugo or a Nebula. In science fiction terms, this is comparable to the fact that the Nobel Prize for literature was never awarded to Mark Twain or Vladimir Nabokov.