If I flip winter festival and my opponent flips valar I choose my plot to go first can I win the game with a total of 15 power while my characters or moribund or do they die first?

You win. The game ends the moment someone counts 15 power. Moribund cards still count their power until Step 6 when they leave play.

If I flip winter festival and my opponent flips valar I choose my plot to go first can I win the game with a total of 15 power while my characters or moribund or do they die first?

Have you had a chance to take a look at the FAQ? It is a daunting document, I know, but a lot of your questions might have the answers you are looking for there. It's very useful when you are looking for a more immediate answer. Perhaps because you are in the middle of a game and the clarification you are looking for may be a turning point in the game.

For example, your question directly relates to the moribund rules on cards leaving play and it has it's own section on Page 19.

http://www.fantasyflightgames.com/ffg_content/agotlcg/support/FAQ-TR-updates/AGoT-FAQ.pdf

Keep in mind that I'm not trying to discourage you from asking questions on the forums here. I just wanted to be sure you at least were aware of the FAQ as it has a ton of information and it might help you answer your own questions.

Also note that it doesnt matter which plot resolves first. Since they both resolve during the same action window, your characters will be on the table and contributing their power when Winter Festival resolves. They'll be moribund if Valar resolves first, otherwise they won't be. That's all the difference.