Sent a conversation message but I’ll just repeat it here that I really can’t find anything about your thoughts on Littlefinger’s potential military campaign for the Vale. I’ve found one response on what you think his goal was but no thoughts on how successful it’d be from a military standpoint given the current state of Westeros and the standing strength of the Vale.

Sorry, I thought I had covered this. 

I could see a scenario where the Vale takes the North easily, sweeps down into the Riverlands aided by rebellion, and then drives on a weakened King’s Landing before anyone can do anything. But I can also see a scenario where the carefully husbanded strength of the Vale is squandered in the North due to the weather, the supply lines, guerrilla resistance, etc. and never even makes it south of the Neck. 

It depends on a bunch of factors:

1. Weather. Given that it’s winter, you could have a scenario where the Vale boards its 35,000 men onto ships to try to land on the east coast of the North and the entire fleet is lost in one big storm. You could also have a scenario similar to Stannis’ where the march from the east coast to Winterfell turns into a death march. Or you could have an easy sail up the White Knife, a quick march to Winterfell, and no problem.
2. Supply. Bigger armies give you battlefield punch and strategic flexibility, but they require huge amounts of food or they start to starve and then disintegrate. And since it’s winter in the North, they’re not going to be able to live off the land. So the Vale would need to bring a lot of food with them and protect their supply lines. 
3. Political support. If the North looks at the Vale as Queen Sansa’s allies come to liberate them from Bolton tyranny, then taking the North shouldn’t be that difficult beyond the logistics of moving manpower. But if the North sees the Vale as its historic enemies come to conquer them, then the Vale’s going to fight a lot of battles it doesn’t want to. Not that it can’t win those battles, but the whole point is to husband its strength and rack up a lot of victories quickly before the real clash.

And all of this then replicates with each Kingdom you’re trying to take. 

Here’s the ultimate thing about the Vale: 35,000 knights sounds like a sound bet, given how weakened and divided everyone is. But there aren’t any reserves after that, and a couple battles that go the wrong way can wipe it off the map completely – look what happened to Jaime’s army and Robb’s army and Stannis’ army and what’s happening to Renly’s army.