It was made of some type of Lightsaber resistance armor. But it was a full blow against his shoulder, thats why he screamed in pain even though it didn't pierce.
Eh, I disagree...unless lucas has specifically stated that Vader's armor was made to repel a lightsaber when struck without extreme precision and force. The whole lightsaber resistant metals/armor like mandalorian iron for example, weren't really introduced until EU. When we saw that glancing hit from luke back in the OT, I honestly think that was a filming error as the reality of the situation was the blow was only intended to graze vader and burn him pretty badly. At the time of the OT, I don't think anything in the universe could repel a lightsaber besides it's equal, another lightsaber.