I don't see the Vale fighting for Cersei. 1) Littlefinger doesn't technically have the power to order the troops down there and how he manipulated Robin into doing it to aid Sansa was Robin's crush for her (which I believe she will use to to bring down Littlefinger) 2) Little finger isn't going to put his army into harms way like that, he's much more likely to wait for the chaos and try to take out whoever is left standing
I also don't see Jon coming down to Kings Landing or her going to Winterfell to ask for his help.
I do think we're likely to see Euron align with Cersei (and there may be a marriage proposal to him in exchange for his help) but ultimately I think it will be Jamie who ends the war by killing his sister.
The wall is broken in the last episode of next season setting up the final season.
I based my thoughts on a few things. If LF is giving up on marrying Sansa, and he still wants to sit on the Iron Throne, well there is a queen without a king right now. I doubt it happens, and its much more likely that Euron teams up with Cersei since he has a navy to combat what Dany has. The show has been working hard at getting everyone in the same place this season. Now they're all in Westeros but the main characters that need to form an alliance are on opposite ends of the continent. I just think its more likely that the northern's get driven south than Dany taking KL and deciding to go up north, unless they can somehow convince her of the WW threat.