I did this for ~Hereticwolf after we had a very in depth conversation about who would win, a dark wizard, or a Sith lord. . . I would like to think this wizard will win . . . especially after reading Harry Potter and the Half-Blood Prince. . .
Since spells can be blocked using objects I'd say Vader has a damn good chance of winning. He would need to bombard Voldemort with objects while coming at him. Voldemort would need to be quicker with his wand than Vader is with his lightsaber.
This.... will be a very difficult fight. A very difficult and very quick fight. It will probably end in at most, 10 seconds, and it will end with an unpredictable outcome. I'm willing to bet that Voldemort can counter the Force with his own powerful magic. Voldemort would have a better range and could probably (and very quickly) form a magic shield that would defend against the Force. Afterwards, he would follow up with an Avada Kedavra and Vader is done.
HOWEVER, if Vader is fast enough, he can close in onto Voldemort before the latter could react quickly enough to counter. Afterwards, all that is needed is a quick slash from an immediately-activated lightsaber (assuming it can be disabled by Voldemort if it had been on in the first place).