That would be my guess. Since the Chinese have learned that they don't have anything at all to fear from Obama, maybe he thinks that a well-armed Vietnam will scare the willies out of them?
What could possibly go wrong? (Considering the Charlie Foxtrot that Obama foreign policy has been over the last 7 1/2 years -- probably a lot!)
Plus also Vietnam is starting to have sucess in the business trade world and that must be upseting China as well.
Plus also China is trying to make their moves because they KNOW Obama is leaving soon, and with Trump most likely going to win the WH come this November, time is not on their side.