These are both remote beaches: Bai Dai is in the far northwest and Bai Thom is on the northeastern coast. A new road to Bai Dai buts down on motorbike time and red dust in your face. You can rest assured that neither beach is crowded.
Both are in military areas, but Bai Dai is open to the public and has a couple of restaurants. The military usually opens Bai Thom to civilians on Sunday but you must leave your passport with the military receptionist while you’re on the base. In any event, do not try to sneak onto the beaches: make local inquiries and obey the rules.