first off, Vader, IMO, wasn't a true sith, he's just another fallen Jedi. he never went through the sith training.
second, why does everyone think that Sidious is the most powerful sith lord? in TOR's era, many on the dark council would laugh at him.
back to the topic...
it's in the novels (ROTS at the end especially) that Vader lost most of his powers after Obi-wan defeated him and was put in the suit. Vader pre "the duel", maybe on all, post, forget it, he gets wasted.