That depends on all of four factors: 1. What spells can and cannot penetrate Vader's armor? 2. How does the lightsaber interact with magical spells? (Can it deflect offensive spells and/or penetrate defensive charms?) 3. What form of Voldemort? 4. What is the setting for the duel? I'm no worshipper of either Star Wars or Harry Potter. I simply find the question intriguing and wouldn't mind trying to logic it out for a bit.
1) parts of Vader's armor were able to absorb a blaster shot, so I'm pretty sure it could withstand at least a single offensive spell on most areas 2) we've seen that spells like Avada Kedavra are stopped by walls and other such obstacles. It would depend on whether these spells are at least somewhat solid or some kind of light, as the latter would most likely defract through Vader's plasma blade. But there are spells that would ignore the lightsaber altogether. 3) let's assume it's book 6 Voldemort, or 7 before he got the Elder Wand. 4) somewhere that neither Vader's nor Voldemort's minions can get to. Could be anywhere, but probably away from any other people.