US can play the spoiler with minimum cost, the enemy of my enemy is my friend. Vietnam can embrace US support, welcome US investments and marines to keep the Chinese in check and US hardly needs to do much but occasionally instigating and plant some ideas to their head, whisper some ideas. The pivot already successful of raising tension in the region.
It's unlikely US would place an active military presence in Vietnam. Notwithstanding the historical bad blood between US and Vietnam resulting from the Vietnam War, anti communist sentiments of South Vietnamese expatriates living in the states, having more American soldiers in the region just after deploying them in Philippines and Australia would be a step too far for China.
Besides, the US is actively engaging Russia in Ukraine which would make EU and Baltic allies nervous if US suddenly increases its commitment to Vietnam. Doing so would not only alienate NATO allies as they are traditionally the top priority when it comes it US supports, but it would also cement Sino-Russian relationship that would make economic sanctions on Russia even more meaningless.
I think the current standoff in South China Sea with Vietnam is a Chinese push back against the renewed US pivot, testing the US resolves in the region since the standoff started right after Obama's visit.
Vietnam has the misfortunate of being targeted because it shares a land border with China which gives China more flexibility should things get out of hand. And Vietnam coast guards forces are more vulnerable as suppose to Japanese or Philippine coast guards. Also, the drill is located in an area that is relatively close to Chinese coast which makes it easier to send in naval and air support should the need arise.
Vietnam is in a bad place right now geopolitically. Vietnam is not close enough with US to warrant immediate assistance. Russia, a traditional ally of Vietnam currently needs Chinese financial and trade backing for Ukrainian crisis.
Btw, things are not going to look too great for Vietnam in the coming weeks, as Russia has decided to send a Pacific fleet squadron to take part in a joint naval exercise with Chinese navy in the South China Sea during Putin's visit next week.