UNCLOS, the Law of the Sea Treaty, defines the rights and responsibilities of nations in their use of the world's oceans. To date, 162 countries and the European Union have joined the Convention. The list includes China but not the US -- not yet.
China is violating UNCLOS when it deploys its oil rig 80 miles inside Vietnam's Exclusive Economic Zone. This is a pre-text to claim the South China Sea (Vietnam's East Sea, Philippines' West Sea), blocking access to one of the most important commerce routes. It is a threat to US's free commerce, securities of US's allies in the area, and US defense in South East Asia.
The US should ratify UNCLOS, then the US will have the legal consistency to punish China if China keeps violating UNCLOS.