vader's armor is just armor.
if you cut it it would bust and you could kill him. the armor isn't for lightsaber protection. jedi and sith don't wear armor for protection, vader has to wear it. it wouldn't be saber resistant.
luke cuts under vader's shoulder blade plate and hits his suit-covered shoulder and it probably did hurt. vader cries out and then yells and cuts through that metal pole because he's pissed. he wasn't being lazy, luke just got a good swipe on him.