Vance's inventiveness is unmatched in fantastical fiction. His works feature hundreds of cultures and races, each more exotic than the last. The worlds he describes - the geography, customs, and often comically predatory relations between the various species - are the product of a sparkling imagination. And it's all portrayed with a deft touch compared with the turgid and heavy-handed exposition other genre authors typically rely on.
Vance's mastery of language was second to none, in or or out of the SFF genres. He played with diction and phrasing like a virtuous on the piano. The wit and interplay in his dialog is some of the funniest I've read - my wife can usually tell when I'm reading Vance because of how often I laugh out loud.
Take the opening lines from The Dying Earth:
Turjan sat in his workroom, legs sprawled out from the stool, back against and elbows on the bench. Across the room was a cage; into this Turjan gazed with rueful vexation. The creature in the cage returned the scrutiny with emotions beyond conjecture.
It's all there - Vance's deliberately overwrought and yet word-perfect diction; the weirdness bordering on the macabre; the wry tone that raises a chuckle.
However, his appeal is narrow. If you don't enjoy exotic inventiveness, arch wit, and a detached irony, then Vance is not for you. The fantasy genre, and its audience, has been veering away from Vance's approach to fiction for decades. Where modern readers want to climb into the skin of a sympathetic character and experience intense surges of emotion and catharsis, Vance's work is told at a remove. You don't inhabit his characters; you observe them explore and scheme and quip their way across a fantastic landscape the way you watch a pantomime of shadow-puppets.
I expect this irony detachment is why younger readers tend to bounce off Vance, in spite of the reverence that superstar influencer GRRM holds him in. His approach is simply at odds with the expectations of today's audience.