It's canon. For whatever reason, Vaders own life-support system is very vulnerable to Force Lightning.
Kind of makes sense, I suppose. If you were the Emperor and you wanted to keep your unruly apprentice in line, you're hardly going to protect him against your strongest attack, are you?
No it isn't. Vader was far more resistant to force lightning than any other Jedi/Sith. What "Canon" proof do you have that he's vulnerable to lightning? The fact he died to it against the most powerful sith lord in the history of star wars?