Honestly, if China sent its navy and some offshore oil rigs into the South China Sea, what could we do to stop them? I don’t see it as something worth shedding American blood over.
Besides, the Vietnamese are clever enough to figure out ways to give China a hard time if China encroaches on their territory too much. Unexplained accidents can happen on oil rigs.
The United States doesn’t have to stick its nose into everything.
Then China dictates all navigation through predominantly international waters. That would be a big mistake.