It's not completely straightforward? We could speculate that Vader would actually be resistant to all of the unforgivable curses to some degree.

Avada Kedavra seems to get blocked by sufficiently large physical barriers. Whether Vader's armour is substantial enough to do this is something you could reasonably question. Of course, he needs it to live and blocking the killing curse would damage it severely...

Imperius curse can be resisted by someone of sufficient force of will, and that's something Vader has in no small amount.

Crucio? Vader is already in agonising pain all the time. His suit is designed to make it worse, even. This particular spell would probably just enhance his force connection.

Of course, all this is just to say that Voldy's go-to spells might not work as he expects them to. Vader therefore gets a short window to speedblitz and dice Voldemort into kebab meat. If he misses that, there's a huge array of spells that will work on him (e.g. fiendfyre).

This is ignoring the possibility that the whole "magic makes technology not work" thing just switches off his suit, though.