While Gaara is likely to keep his distance with his attacks at first, but I'm pretty sure Vader can close their gap with his superior speed, while Gaara can boost his speed as well via body flicker, Vader's precognition will make up for it and simply counter the speed boost Gaara's gets in short bursts. Also, Vader's lightsaber can and will cut through Gaara's Auto defence, as lightsabers have shown to casually cut through the hauls of ships with High 7-C durability. also, there's also the fact that Vader's lightsaber could turn Gaara's sand to glass making it ineffective before someone says Gaara's sand is extremely resistant to fire. Plasma> Fire
Vader is likely to go in and cut through Gaara's defence but Gaara can easily avoid the first few strikes with substitution. Gaara isnt dumb his going to know that Vader's lightsaber is a problem and try to keep vader at a distance with Sand clones,sand tsuname's or attempt to seal via sand pyramids, however vader can simply blast the sand away with the force, create force barriers around himself or cut down any sand around him, Also Vader could telekentically pull Gaara towards him and cut him down with his lightsaber, the only way for Gaara to counter this is to create his sand mom or hands to hold him in place to avoid getting pulled in, alternatively Gaara could simply try and fly out of range but I doubt this would work when even canon Vader has rippped starfighters from the sky with ease and im willing to bet starfighters are heavier than Gaara.
even if Gaara somehow manages to disarm Vader he will eventually have to deal with his force choke, in which Gaara has no counter. it's literally invisible and intangible hands crushing your Throat, Gaara would have to try and damage Vader while his suffocating dying...which I find unlikely with Vader's superior speed, also Vader can do this at even farther distances, as his force choke was capable of effecting someone thousands of meters away
so after a long fought battle, I think Vader takes this
I agree with @Shadow for pretty much everything except one thing. Gaara's sand isn't being turned into glass. Amaterasu is so far above normal fire that it isn't even funny (Nor does it make any logical sense). Gaara's sand wasn't turned into glass by Amaterasu.
Amaterasu biggest thing is that it keeps burning and can ignore durability to an extent, for its hype ameterasu isn't that hot, it's barely melted steel and struggled to melt through Large chunks of ice.
There's no reason to assume it wouldn't just because no one is wacking their lightsaber in the sand, it's like me saying a fireball jutsu cant burn Staw because there's no canon part where they stand and burn tree's Straw down.
with the exception of being extremely resistant, Gaara's Sand is still normal sand, it's just being controlled remotely via chakra
though to be fair it's far more likely the sand would get vaporized than actually simply turning into glass