If you're running a tube, put it over the vent hole and lean SLIGHTLY towards the beach. Plunge, cover the air vent, and pull. Repeat in the same hole if it's deep and you didn't get it.
If you're digging, dig 6" to the surf side of the vent hole. Easiest when you're out far enough to keep things wet. Dig a hole elbow deep 'behind" the clam, big enough to reach into, without it collapsing. If it's wet, the clam will just fall into your hand.
If you see a vent hole ring, it's there. They maintain them with every wave.