[SampleMan]

I’m not seeing any reason to prop up Vietnam. One of the worst possible scenarios for China would be an occupation of Vietnam, a country that has been hostile to China since the Ming dynasty.

[Joe Brower]

Iirc, when China invaded Vietnam in 1979, in a month they lost over half the number of men that the U.S. had killed in over a decade of war there.

Of course, the way China values human life, that's a pittance, but any conflict there would certainly be very costly, and keep them busy for a while.