Example, I have 14 power and reveal plot X and my opponent reveals Valar, and I have Victarion Greyjoy in play. Vic will get his response power while Moribund. Do I win the game?
Can you win by receiving a moribund power?
Sure. Why wouldn't you?
Nothing is stopping you from using the ability while he is moribund and the rules say you win as soon as you get 15 power. There is no condition of "stability" or "longevity" of the power. Nothing says you can only win at the beginning of an action window.
People do this with Red Wedding all the time. They are at 13 power and control the only Lord and Lady in play. The Lord has 4 of those 13 power on him, the Lady has 3. Even though one of them dies from the same plot effect, the player gets to 15 while all those card - and power - are still in play.