Why didn't Darth Vader's shoulder not become severed when Luke Skywalker hit him with his lightsaber?

Not hard enough hit to cut the whole arm I reckon. : )

Glancing blow. Plus, it’s hard to say where there is meat and where there is metal.

Cuz he ain’t no wuss like Frodo. He’s a badass like Darth Vader.

Coz they too heavy man, he carries the weight of the world on his soldiers shoulders, a common theme with all megalomaniac would be dictators.

What? You found an inconsistency within the rules of the Star Wars universe?

I’m shocked. Shocked I say.

Luke was not trying to kill him.

Because ole’ Darth, as black as he was, was not The Black Night.

…or the Black Knight.

Or it is his Armour that is resistant to blaster and light saber.