If the lords of the Vale had somehow managed to work around Lysa and ride to help Robb, would they be doing that as part of the old RR alliance or because they implicitly sought to join his kingdom? Robb is Lord of Winterfell in his own right and he can plausibly claim some sort of dominion over the Riverlands through his Tully grandfather, but the only way to cement a new kingdom that includes the Vale would be a marriage, probably to Ysilla Royce (not yet married to Mychel Redfort).

That’s a major quandary, and part of what would have made helping Robb a bit tricky. But again, it depends about timing – if it’s before Catelyn XI, then it’s pretty straight-forward. The Vale hates the Lannisters because they think Jon Arryn was murdered by them and the theft of the Warden of the East title, Robert Arryn’s grandfather is being besieged, his cousin’s father has been imprisoned and his cousins (both Robb and Edmure) need military assistance, and Tywin’s breaching the King’s Peace. 

If it’s after Catelyn XI, then it gets more complicated. The Vale has never been part of the North and isn’t about to bend the knee to Robb – if they’re sufficiently alienated from the Lannisters, they might proclaim Robert the Falcon King. Only a federation of equal kings is going to work for the Vale. Now, it helps that Robb and Robert are cousins, but a marriage would work to really tighten those links- Robert’s hand is free, first cousin marriage isn’t considered incest.

On the other hand, the Vale is notably conservative and loyalist when it comes to the monarchy – even when it came to Robert’s Rebellion, a good number of Vale lords rose up against the Lord of Eyrie rather than war against the Targaryens. I doubt that the Vale would be as punctilious with Joffrey, especially once Stannis’ letter is known. 

On a third hand, Robb’s political strategy might change depending on whether the Vale lords are present for the war council in Catelyn XI. The Vale lords would probably not be ok with an outright secession, and Robb is smart enough to know that you don’t alienate 35,000 swords. So Robb might hold back from declaring any way for the moment, while pursuing his war against the Lannisters.