An economic threat backed by military power doesn't require a military response. We and the Vietnamese have all sorts of potential economic responses, although

An economic threat backed by military power doesn't require a military response. We and the Vietnamese have all sorts of potential economic responses, although of course our's are much greater, including the "nice pile of US Treasury debt you have there, it would be a shame if we declared it worthless to anyone who might buy it". And the PRC's actions are already getting its neighbors to band together, even with Japan (!). Or there's the convoy approach, we task some ships and subs to escort convoys of merchant ships to and from Vietnam, following our long freedom of the seas policy. We're already sending the occasional Arleigh Burke class destroyer into the new areas of the ocean that the PRC is claiming.