But would the River lords still acclaim Robb king? I assume the Vale would want it’s own independence, but would they divvy up the Riverlands with the North?
The Vale coming in on Robb’s side really changes the politics of that post-Camps council at Riverrun, because they would be a large contingent of socially conservative lords who don’t owe Robb a debt in the same way that the Riverlords would, and Robb would need to keep them onside because they’d be a plurality of his army.
Does the marriage pact with Walder Frey still happen in the scenario? Robb would have to tie the Vale somehow.
Depends on when the Lords of the Vale arrive. If it’s somewhat late, Robb makes the marriage pact to cross over the Twins and attack Jaime’s army. If it’s early, Robb may decide to try to catch Tywin between their two armies instead, thus no marriage pact.