Do you have any idea how large China's population is?
If a billion people vanished from China tomorrow it would still be the second most populous country in the world.
The Chinese military has about 2.5 million active + reserve personnel (a number that shrinks with modernization). Out of a population of 1.39 billion, that's 0.18%.
The Vietnamese military has 482,000 active + 5.5 million reserve (WTF is up with that? Do you just give anyone a rifle and a week of training and he's now a "reservist"?
), call it 6 million. Out of a population of 95.5 million, that's 6.2%.
China and Vietnam aren't even in the same order of magnitude when it comes to drawing recruits from the population. All of this is meaningless, however, since China would use its overwhelming technological, operational, and doctrinal advantages to cripple Vietnam's ability to fight and sever its supply lines before it could even think to begin mobilizing.
Duterte doesn't seem all there at times, but I doubt even he is addled enough to believe Pompeo.