Not that the US involvement are a bad thing, but I am reserved to this decision. My opinion is, with US involvement, it's only get more complicated.
First of all, no ship should be harassed by any government in the open sea. YES, EEZ is STILL INTERNATIONAL WATER. If you think it's your EEZ, you can board them and turn them around. NOTHING MORE.
Firing a flare and set fire to the ship in international water (AGAIN, EEZ is still international water) is the same as restricting freedom of passage. No country should be allowed to do this, be it Vietnam or China.
With US involvement, it only step up the chaos and increase the risk factor on the "Chance" that all gone wrong. Sure, we need those channel open but you cannot simply depend on the US to check on the PLAN. It's not gonna work.
What SCS need is some sort of Anti-Piracy gang like Gulf of Eden doing now. A multi-national anti-incident patrol. We probably need to see more of the action of Japan, Australia, India and New Zealand more than we need to see more action from the US.