Garth Vader could work very nicely. Especially if everytime anybody says "Garth Vader" we hear Wayne off camera saying "Party on Garth".

However, correct me if I'm wrong but...

"A young Jedi named Darth Vader, who was a pupil of mine until he
turned to evil, helped the Empire hunt down and destroy the Jedi
Knights. He betrayed and murdered your father. Now the Jedi are all
but extinct. Vader was seduced by the dark side of the Force."

...is the only time the name "Darth Vader" is used in ANH. Darth on it's own is used two more times...

BEN: Only a master of evil, Darth.

BEN: You can't win, Darth. If you strike me down, I shall become more
powerful than you can possibly imagine.

So whatever the gag is, it's only gonna play 3 times. Lots more Darth in RSB and ROTJ I think.