Shantanu is a very crafty person who executes his plans slow and steady. But pea brained Vish and her frog faced grandma cannot understand his plans. Thus they fell for it. He would not help Vish if it was of no use for him and even more his plans are long termed. Thus, he would not let his plans expose so soon.
He would probably help Radev to throw Vishaka out as he is holding the reins of the whole issue and Vishaka would be too late by then.