Darth Vader is physically and emotionally crippled. All four limbs are artificial, he can barely breathe without a respirator, and he appears to have sustained severe spinal injuries.

However, with the Force, all things are possible. Vader can survive perfectly well without his suit when he meditates on the Dark Side, but he is unable to sustain it because the measure of joy and self-esteem it grants him breaks his concentration. Vader is held back to a great extent by the same psychological trauma that he draws upon. Vader's self-loathing fuels his power in the Force.

Force lightning is a warping of the Force, a projection of pure energy; usually of the Dark Side. In RotJ, Sidious' overwhelmingly powerful lightning destroys Vader's life support. However, Vader can resist it to a certain extent, as shown in the Force Unleashed.

Despite this, Vader cannot employ Force lightning.

It is my theory that this is more of a psychological than physical block. Vader has the power and control to theoretically channel lightning away from his circuitry. However, to use it himself, he would have to draw from the Dark Side - that is, his own self-hatred. If Vader were to use a pure manifestation of the Dark Side, his own demons would cause it to rebound upon him.