DominicCobb said:
Just means he knows an ability Vader does not.
I wouldn’t even say “knows” as it implies it can be learnt. Emperor simply has an extremely rare given ability/potential that Vader does not. It is like getting an impossible mutation of an eye that allows eagle eye quality sight. Those who don’t have such mutation simply cannot have that kind of sight ability, not matter how hard they may try to “train” their eyes.