A bit late, but still :)
There are three words that adresses people in different ways in this game.
House, Player, Opponent. (in this specific scenario, the difference between opponent and player is crucial)
Judging from the Bronn Vassal House Card. "After you reveal house cards, your opponent may choose to discard 2 of their available tokens to reduce this card's combat strength to 0"
This is of course to address the person you are playing against, and not the house (ie possible vassal) as the vassal cannot have power tokens.
Even more clear would be the example if two vassals are in combat. The vassal playing against Bronn doesn't have power tokens (ie, the person commanding the vassal needs to pay, meaning the opponent is 1 player with 2 or more houses)
The rulebook states that you as a commander of a vassal resolve the combat on behalf of the vassal. - meaning that you do everything, get the rewards and the punishments. That makes the vassal a non-player imo.
Finally, I think the text on Walder Frey nullifies any confusion as it has both player and opponent in it.
"Any player (other than your opponent) who grants support to your opponent must grant that support to you instead."
So the card would not reverse the support of your opponent's own vassal. In fact, it is the only counter to the situational, but sometimes powerful, card of Walder Frey. :)