So Voldie (yeah, that’s what we call him around here) doesn’t know about the whole cybernetic thing (Vader’s dependency on his mechanically augmented body for survival), and Vader has to deduce during the fight itself that Voldemort needs his wand/ability to breath in order to cast spells. Voldemort probably would have some ability to defend himself from the force using spells, but … he needs his wand. And he can mostly only keep track of what’s in front of him.
Voldemort might – just might – be smart enough to think, “Oh, gee, shiny blade comes from that thing.Expelliarmus.” But then he has an angry telekinetic sith lord to deal with instead of an angry sith lord with a lightsaber. Plus, the lightsaber just becomes one more thing Vader can move telekinetically. So we’re talking an angry sith lord with telekinesis and a flying laser buzz-saw.
Voldemort has the advantage of mobility, Vader has a certain machine-like relentlessness. And, he’s reasonably intelligent. So I’m guessing, wand goes bye-bye eventually (if only because all the blasts coming from it makes it look like a weapon). After that, Force Choke. Or suckerpunch to the face. I bet that would hurt.
On the other hand, Voldemort might guess at Vader’s cybernetic nature, but it’s unlikely . . . given that there aren’t any cyborgs in his world. Wormtail doesn’t count. 😉
Force lightning won’t happen (Vader never does it in the films because he can’t – you need living hands in order to do it, and once Obi-Wan cut them off he lost that option); Force vampirism might, but you may need hands for that, too, so who knows?
Crucio? Vader’s dealt with pain before. . . what with the lava, and the constant pain from his implants and whatnot.
Avada Kadevera? Now, that could be a problem, but the force is more likely to give Vader the ability to resist that than the average Wizard in Harry Potter seems able to. Wizards who can’t defend themselves from spells . . . bad idea. Plus, Voldemort needs to be able to concentrate in order to cast it, and it’s hard to concentrate when you’re being hit by rubble from multiple sides.
Advantage? Vader. Most likely victor.
Aftermath: Empire kills off any Death Eaters who don’t join up willingly, probably wipes out most of the wizarding world. If it doesn’t work the first time, Star Destroyer blasts it from orbit. Repeatedly. This is Hogwarts:
As a result, Harry Potter and his few remaining friends (naturally, the sole survivors of the debacle) join the Rebel Alliance and Luke teaches them how being a Jedi works. Hermione figures it out first. They defeat the empire with a lightsaber in one hand and a wand in the other. Dumbledore joins Old Ben, Aniken, and Yoda when Luke has his vision at the end of RoTJ.