Why Yohn Royce wanted the Vale to go to war against the crown?

[Mithras]

Did anybody notice the family ties of Freys to the Vale?

Perra Royce is the first wife of Walder Frey and she is the mother of Stevron, Emmon and Aenys.

Stevron Frey's second wife was Marsella Waynwood. She is the mother of Walton Frey whose wife is Deana Hardyng.

Edwyn Frey's wife is Janyce Hunter.

We see that the primary branch of Freys have important ties with the Vale Houses. We also see such ties in the Crakehall Freys too.

Ryella Royce is the wife of Arwood Frey, son of Hosteen Frey.

Carolei Waynwood is the wife of Geremy Frey. What is more is that their son Sandor Frey is a squire to Ser Donnel Waynwood and their daughter Cynthea Frey is a ward of Lady Anya Waynwood. Pretty big deal these last two are. Carolei must be very close to Lady Anya so that she took her daughter as a ward and her son took Carolei's son as a squire.

In short, soon after the Freys declared for Robb, it is natural for some Vale Lords to want to join Robb even only with these Frey connections. As we know, there are even closer ties between the Starks and both Arryns and Royces.

[TallTyrion]

Did anybody notice the family ties of Freys to the Vale?

Perra Royce is the first wife of Walder Frey and she is the mother of Stevron, Emmon and Aenys.

Stevron Frey's second wife was Marsella Waynwood. She is the mother of Walton Frey whose wife is Deana Hardyng.

Edwyn Frey's wife is Janyce Hunter.

We see that the primary branch of Freys have important ties with the Vale Houses. We also see such ties in the Crakehall Freys too.

Ryella Royce is the wife of Arwood Frey, son of Hosteen Frey.

Carolei Waynwood is the wife of Geremy Frey. What is more is that their son Sandor Frey is a squire to Ser Donnel Waynwood and their daughter Cynthea Frey is a ward of Lady Anya Waynwood. Pretty big deal these last two are. Carolei must be very close to Lady Anya so that she took her daughter as a ward and her son took Carolei's son as a squire.

In short, soon after the Freys declared for Robb, it is natural for some Vale Lords to want to join Robb even only with these Frey connections. As we know, there are even closer ties between the Starks and both Arryns and Royces.

There's so many Freys it's hard to find a house in the south they aren't connected to in some way.

[Irish]

Yohn was related to the Starks, his Liege was related to the Tully and Starks. The Vale, by all rights, should've joined the war. If they'd met up with Bolton before he engaged Tywin, the battle would've likely went against Tywin rather than resulting in Roose's defeat.

Bolton had 18,000 men, Tywin had 20,000 but the Vale could've easily brought 20,000 men to link with Bolton. That 2-1 advantage would've crushed Tywin, or at least forced him to retire to Harrenhal.

That, and the fact that if they had Tyrion captive (rather than Lysa releasing him), would've pushed the war in their favour. Jaime and Tyrion could potentially have been held captive.

[Mal Malenkirk]

Aside from all the possible rationalization proposed above, I would add that Yohn Royce is a celebrated warrior who fought in many battles. War can be seductive to some men, especially those who are good at it and derive most of their status from it. All of Royce fame is derived from him being a great warrior. Beside his honor, what positive trait does he have that are not martial? I don't remember reading about him being a great administrator or a wise judge, only that he is a badass fighter. And what is a warrior without a war?

A man like Yohn Royce might subconsciously yearn for one more battle while he's still fit and strong and therefore be willing to act on pretty thin ground if a half valid rationalization for war is presented. Fighters gonna fight.

[Mammoth Slayer]

A lot of excellent reasons have been put forward for Bronze Yohn and the knights of the Vale going to war against the Crown, however when it comes down to it people really act from self-interest. Going to war would be risky for the Lords of the Vale, as if they ended up on the losing side their lands could be confiscated and passed onto other houses. Would they have much to gain from going to war? If they invaded the North, they could become embroiled in a war with Bolton and/or Stannis. Ruling the Northmen would not be easy. If they took the Riverlands from the Freys, would the Tulleys see them as rivals or allies? Considering the strengh of the Lannister/Tyrell alliance I think that Lysa's policy of neutrality may have been a wise course as far as the interests of the Vale are concerned.