I think Littlefinger's smirk was one of confidence. He has one of the most dangerous assassins in Westeros on his team. Littlefinger is still working Sansa, and he has a few good cards to play. First, Sansa can never say that Littlefinger killed Lyssa Arynn because she would be admitting she lied about it. Secondly, Littlefinger has saved Sansa's life, twice (escape from Kings Landing, and stopped Sansa's nutty aunt from tossing Sansa through the Moon Door). Finally, Littlefinger all but said he doesn't give a about the White Walkers. Either they will eliminate humanity, or they won't, and Littlefinger would be more than happy to let Jon Snow take off on a suicide mission to the North, preferably with some dragons. Littlefinger is in pretty good shape in the game, which is a sure sign he is about to get killed.
I am a little confused by the size of the relative armies. At this point in the war, the Vale should be one of the biggest armies left. The North, Riverlands and the Lannisters should have been bled dry by now, since they were in the middle of all the fighting to date. The Reach and Stormlands have engaged in multiple battles too. The Stormlands were on both sides of Blackwater Bay, and were the bulk of Stannis' army after that. Dorne and the Vale have basically sat out the fighting. But Davos said they only had 10,000, which would put them in a distant 3rd place behind Cersei and Dany. Even assuming Cersei is getting some extra support for being Queen and is leveraging the Reach, her forces should still be in shambles.