Darth Vader isn't a Jedi Knight. His very name means that he is
a Sith Lord not a Jedi Knight. From the second Anakin Skywalker is
dubbed Darth Vader by Palpatine Anakin Skywalker ceased to exist
and Darth Vader was born. That was also the second that Anakin was
no longer a Jedi Knight, he was officially a Sith Lord. So your
question is faulty.