Wouldn't it be awesome if Martin was just trolling us all and was actually nearly finished and released the book this summer?
I can dream, right?
I'm hoping they announce a publish date during or shortly after the upcoming season... which means we'll see the next book around Christmas.
Although that is highly unlikely i think its a good sign we haven't heard much from the Martin camp in awhile.
As far as the Vale being impregnable... I can see it falling to Dany's dragons and her army making a stand against the white walker's there. Plus you have Tyrion's relationship with the mountain men they can eventually use to add to their forces.