In Empire, Luke swipes at his armor, and he seems to yell in pain, but it obviously does not sever or cut his arm. In Return of the Jedi, we see Luke slice off Vader's hand. Is he saber proof or not?
The only detailed information about lightsaber proof parts we have are Vader's gloves. They were (According to "Darth Vader's suit" on Databank) indeed made with micronized Mandalorian Iron that was proof against lightsabers and (as seen in Han shooting him on Bespin) blasters.
However, according to "Star Wars: Darth Vader: A 3-D Reconstruction Log", the armor is mostly made from an obsidian/plasteel composite, tempered by Sith alchemy. Doesn't say if that makes it lightsaber proof but presumably proivides a modicum of protection.
That protection DEFINITELY wasn't absolute - aside from Luke chopping off his arm in RotJ, we also see an episode in "Dark Lord : The Rise of Darth Vader" novel by James Luceno when a Jedi (described by Vader as "unremarkable", so RotJ wasn't just about Luke's special power) cuts through it:
In his personal quarters aboard the Exactor, Vader examined the damage the Zabrak lightsaber had done to his left forearm. After assuring himself that the pressure suit had self-sealed above the burn, he had peeled off the long glove and used a fine-point laser cutter to remove flaps of armorweave fabric that had been fused to the alloy beneath. The Jedi's lightsaber had sliced through the shielding that bulked the glove and had melted some of the artificial ligaments that allowed the hand to pronate.