The one obvious thing they had in common was that they were genre-jumping authors. Jack Vance began as a pulp sci-fi writer in the Forties, but he was also an award-winning mystery/crime writer - indeed, Vance often asserted that he wasn't really a science fiction writer at all. (The only TV adaptation of a Vance story, Bad Ronald, is not science fiction, but does emphasise the power of fantasy.) Iain (M.) Banks enjoyed mainstream literary success but also wrote popular science fiction, something many 'serious' critics obviously found hard to deal with.
Vance influenced Banks in obvious ways. The sheer flamboyance of the former's invention is apparent in the latter's Culture series. Vance was not the first sf writer to create a consistent future universe (that of the Alastor Cluster and the Demon Princes series) but his was one of the biggest and brightest. Banks' Culture and various rival powers were just as flamboyant as Vance's galactic realms.
Both men were strongly atheistic in outlook, and often cast a sceptical eye at the way religion prompts people (and other beings) to do bizarre and brutal things. And both had a wry sense of humour - neither was at all po-faced. There's a sense of the absurd about many of their invented cultures (as well as in The Culture), and there is an emphasis upon games, gambling, sport, and related matters. These recreations often have a lethal side: there's Banks' game of Damage, which can burn out the brain, while in The Dirdir Vance depicts the hunting of humans by aliens in an enclosed ecosystem called The Glass Box. It's worth noting that in both cases people volunteer to take part.
They were very different, though, in other ways. Vance was essentially conservative in outlook, and his books reflect traditional attitudes towards gender roles - not surprisingly, as he was born in 1916. Banks' Culture novels depict a world where genetic and other biological modifications are normal and desirable, and his post-war Scottish origins led - not surprisingly - to his embracing feminist and generally progressive views.
Vance's invented worlds, for all their bizarre customs and beliefs, are often modified forms of 'traditional' civilizations, based on a money economy and fairly familiar social class distinctions. On the few occasions when he describes a highly governed or socialistic society it is clear he doesn't think much of it. In Wyst: Alastor 1716, for instance, an 'egalistic' society is depicted as economically hopeless. (Vance is just as hard on stratified cultures propped up by religion, a stance taken to extremes in the excellent Emphyrio.)
By contrast, Banks' Culture is techno-socialist and run - most of the time - by extremely intelligent machines, and in The Player of Games he sets it against an alien space empire that embodies all the worst traits of modern Britain. He was clearly fascinated by militarism and the paraphernalia of warfare at all technological levels, but often describes militarists with a kind of jovial contempt.
One very obvious common thread in the books of both men is the way that (again, very traditionally) they tend to focus on the saga of one key individual to illuminate the worlds they invent. In Vance there is usually a quest to set out upon or vengeance to be wrought. Things are not always so clearly defined with Banks, but damaged or marginalised characters are more likely to set things in motion. The authors were both civilized, generous men who offered their readers bizarre, dangerous worlds and ruthless heroes: they will both be missed.