ian mcdiarmid sucks in esb just because he looks bloated and catatonic, and the fact he has to tell vader that the skywalker hes looking for is his son, what? also the fact that in the prequels it was established that there could only be two sith, why would the emperor send vader out to get luke when vader would try to convert him (which he did) espescially since vader was related to luke, what a gamble for such a master designer of everything in the PT