As China seeks to project naval power, the last thing it needs is a formidable landpower on its doorstep that can withstand Chinese landpower. The Soviet Union and the Kaiser's Germany both foundered in their naval ambitions by their need to also field powerful armies to resist land enemies.
So when a US ship visits Vietnam, this is a good sign that in time we might have a tough ally on China's doorstep. With an army able to resist what China can throw at it, and naval and air bases able to flank Hainan Island, threaten China's interior, and interdict the South China Sea, Vietnam would make a wonderful asset to box in communist China. And since the Soviet Union went belly up and now sucks up to Peking, Vietnam could use a powerful friend.
Even without Vietnam, we have pressure points to fight China effectively. But having an ally that can inflict ten- to twenty thousand KIA on China in a month of fighting without even committing their first line divisions is an asset.