I don't think that Darth Vader is immune to effects of Force lightning. Galen Marek did overload the defenses of Darth Vader's armor with his lightning at one point.
Darth Sidious is superior master of the dark side then Galen Marek so the former is more specialized and effective in matters of offensive applications of the dark side accordingly.
Vader's missing hand isn't such a big deal, it was not as if Sidious directly bombarded Vader with lightning, Sidious was not in this position. Vader picked Sidious from behind which led to some of the latter's lightning bolts hitting Vader erratically and this was enough.
In a normal setting in which Vader is fine and Sidious unleashes his barrage of lightning on Vader in proper manner, I don't think that Vader can last.
Originally posted by Emperordmb
He had a missing hand against Starkiller as well, so that's not really an answer.
Not in TFU II when he tanked all the Lightning from him.
Honestly these are just a tiny fraction of the inconsistencies we'd get from the previous EU. Which is why the EU has been revamped.
Vader is again susceptible to FL in the new canon (without catching it in a Lightsaber) as far as we know.
Originally posted by DARTH POWER
Not in TFU II when he tanked all the Lightning from him.Honestly these are just a tiny fraction of the inconsistencies we'd get from the previous EU. Which is why the EU has been revamped.
Vader is again susceptible to FL in the new canon (without catching it in a Lightsaber) as far as we know.