Krishna wouldn't have stopped Ram from accepting Vanvaas as that was a necessary Leela to cause Ravan's demise. By going to Vanvaas, Ram was actually offering a bait to Ravan in the form of Sita, which in turn would give Him an excuse to kill Ravan. Ravan tried to trap Ram in his Maya by sending the Golden Stag, but instead himself got entrapped in Ram's Maya i.e. Sita and invited his own downfall. Krishna Himself did this Leela several times, e.g. by fleeing from battles with Jarasandh and relocating to Dwarka.
But, but, but. I think Krishna would have never accepted Ram's treatment of Sita at any cost. He would have never allowed an innocent lady to jump on fire or banished a pregnant woman to please few jackasses. He would have argued, quarreled, turned against the whole world and ultimately would have succeeded to secure justice for her.