Two words. Avada Kedavra.
Lord Voldemort could win in the first few seconds of the fight. He would just say "Avada Kedavra" and Darth Vader would die. And even if Darth Vader could strike him in those two seconds, what good would it do, Voldemort can only be killed if his horcruxes are destroyed.
Darth Vader will forever rule the universe.
I don't think Voldemort's wand can stop a lightsaber.
Darth Vader has the force, a lightsaber and is the one who can bring balance to the force. He can block Voldermort's death spell with his lightsaber and then force choke him. DONE!