Here's a blurb which describes the Phantom and illustrates the similarities between the two characters:
He’s a character of superhuman power and strength, clad in black leather and a flowing cloak, with a stylized helmet to cover hideous scars on his face. The damage to his vocal cords means he breathes with an eerie rasp, and has a box with lights and switches on his chest to help modulate his voice. He retreats to an egg-shaped chamber where he can meditate and find peace. He’s not wholly evil, but has been enslaved by someone who is: a cruel master who exploits his unique powers in order to further his own, evil ends.
DePalma's Phantom of the Paradise came out in 1974, he conceived of the story well before it was released, and Lucas and DePalma are close friends.
So, is Darth Vader a Phantom ripoff? Lucas ran with a different idea, and created a more fully realized character so I wouldn't say it's a total ripoff. But did the Phantom influence Lucas' "vision", and help bring Darth to the big screen? No question.