I am playing a joust game and opponent A does a power attack on myself. I unoppose the challenge and then play Red Vengeance and choose him as the opponent.

Does player A still win one power for unopposed ?

Yes. He is still the winner of the challenge. You are still the loser of the challenge. You still counted 0 STR for the challenge. All Red Vengeance does is change who fulfills the claim effect "as if" they had lost the challenge. It does not define a new winner or loser (the way that, say, Misinformation does).

Thanks ktom for all your answers today, appreciated