Can atrial fibrillation with rapid ventricular beat cause ventricular fibrillation?

Not usually. Not typically; however, if you also have an accessory pathway (wolff-parkinson-white syndrome), this is a unique situation where af is known to cause very rapid ventricular activation during af, which can lead to vf. Rapid ventricular response may cause cardiac weakening over time, which in turn may increase the risk of vf.