And it would be more like, riddle I am your father..... oh wait he's dead!
Easy situation. Here's how it'd go:
Darth Vader comes charging at Voldemort
Voldemort pulls out his wand Avada Kedavra bitcch!
I think Voldemort would win but I want Darth Vader to win.
Okay, Voldemort would pwn Darth Vader any day. Especially cuz Darth Vader would be like "Tom, I am your father." And Voldemort would go "I THOUGHT I KILLED YOU!!!" and kill him(;
of course, darth vader would run to him and voldemort would float and kill him from above