I read somewhere that Prowse's relationship with Lucas was bad. He had given spoilers away about The Empire strikes back while the movie was being shot.
It is said that, since Lucas then did not trust him anymore, Vader's line in the script became "I killed your father" and that it's the line Prowse did play, and no one knew about the real line, not even Hamill.
If I recall well, that's from an interview with someone from the staff I saw on yt. Being given wrong text, being re-dubbed, not being Vader's face when he is unmasked, all that had pissed him off.
I didn't realise the voice of J.E. Jones is a bit pitched for Vader's voice. In French his voice is even deeper.
https://www.youtube.com/watch?v=UQVV-IW2Vsc&t=1m39sAnd yeah, no Darth Vader around here, it's Dark Vador -_-