Forget the vents, it's not them.
Is the water level in the tank proper? Have you changed the flapper valve? A bowl gets refilled partly by any water that does not drain out during the flush, but primarily by what comes out from the hose emptying into the overflow tube. If that tube has a kink, or there's crud in the line to it, or the flapper doesn't open long enough, the flush can be compromised as can the eventual water level in the bowl. Also, if you changed the flush valve, it may not have the proper balance of flow between the tank and the bowl so the bowl may need more by the time the tank is full...it's a balancing act. The older ones overfilled the bowl, but the excess went down the drain...can't do that anymore and stay within the 1.6g mandate - both the bowl and tank need to be filled at the same time and balance is critical. On yours, seems something disrupted that balance.