I think the issue with Ake is he's quite short so that might be a problem when defending set plays - Gomez is pretty tidy in the air. Also Ake is pretty exclusively a LCB, and while I have no doubt VVD could play on the right that'd be a non-insignificant overhaul.
For me Ake is a nice squad player in that he can play CB, LB, or in defensive midfield, but I'd hesitate to call him a real game-changer at any of those spots. It's pretty significant to me as well that Chelsea have turned down his buyback option despite ostensibly needing reinforcements at those 3 positions.