The Vietnamese hate the Chinese (1K years of occupation) so we shall see.
Those missiles would allow the U.S. to play China at its own game: dangling a military veto over the transit of Chinese commercial and military elements through the South China Sea if China attempted to do the same to the U.S. or its allies. The purpose here would not be to instigate a conflict with China but rather to deter China from doing that which it wishes to do. Namely, using its island construction and militarization campaign in the South China Sea to economically and diplomatically pressure regional nations to kneel to its influence.
More @ Washington Examiner