Not a stupid question at all.
There were substantial numbers of Vale lords who wanted to fight the Lannisters for several reasons:
“Lord Jon was much loved, and the insult was keenly felt when the king named Jaime Lannister to an office the Arryns had held for near three hundred years. Lysa has commanded us to call her son the True Warden of the East, but no one is fooled. Nor is your sister alone in wondering at the manner of the Hand’s death. None dare say Jon was murdered, not openly, but suspicion casts a long shadow.” (Catelyn VI, AGOT)
“Pycelle pushed himself to his feet. He was clad in a magnificent robe of thick red velvet, with an ermine collar and shiny gold fastenings. From a drooping sleeve, heavy with gilded scrollwork, he drew a parchment, unrolled it, and began to read a long list of names, commanding each in the name of king and council to present themselves and swear their fealty to Joffrey. Failing that, they would be adjudged traitors, their lands and titles forfeit to the throne.
The names he read made Sansa hold her breath. Lord Stannis Baratheon, his lady wife, his daughter. Lord Renly Baratheon. Both Lord Royces and their sons.” (Sansa V, AGOT)
“Lysa was as lonely as she was. Her new husband seemed to spend more time at the foot of the mountain than he did atop it. He was gone now, had been gone the past four days, meeting with the Corbrays. From bits and pieces of overheard conversations Sansa knew that Jon Arryn’s bannermen resented Lysa’s marriage and begrudged Petyr his authority as Lord Protector of the Vale. The senior branch of House Royce was close to open revolt over her aunt’s failure to aid Robb in his war, and the Waynwoods, Redforts, Belmores, and Templetons were giving them every support.” (Sansa VII, ASOS)
The Lords of the Vale don’t declare for Robb Stark because Lysa has summoned her knights to the Eyrie and refuses to let them leave – “If it were up to me, I would take a hundred men into the mountains, root them out of their fastnesses, and teach them some sharp lessons, but your sister has forbidden it. She would not even permit her knights to fight in the Hand’s tourney. She wants all our swords kept close to home, to defend the Vale” – although given her close cooperation with Petyr Baelish later in ASOS, we can surmise that the reason why the knights of the Vale were kept at home is that A. Littlefinger didn’t want Robb Stark to win the War of Five Kings, and B. he wanted to keep them fresh for his attempt to win the Iron Throne.
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