SilverWook said:
Peter Cushing's ghost would like to have a word with you. ;)
Lol!!
It is my guess that, while the Death Star was being built, Vader and Tarkin found a mutual respect for each other and evil. Maybe Tarkin befriended Vader in a way no other human or Sith could .... maybe even did him some solids ya know. Then again, maybe the Emperor commanded Vader to defer to Tarkin due to his own plans for the future.
It's anyone's guess.