Question: Despite millions of Vietnamese having died during the Vietnam War, the Vietnamese have nevertheless become highly pro-USA. So why, despite only about 35,000 Vietnamese dying in the Sino-Vietnamese War, have the Vietnamese become so anti-Chinese?
Answer : Well, I think that your question is complex and also not related to each other between China and America.
Firstly , I do not understand your logic because Vietnam-American relations and Vietnam-China relations are different stories. How you could use the amount of death between the American war (Vietnam war in America) and the Vietnam-China border war 1979 to indicate that Vietnamese is pro-China or Pro-America. You should learn more about Vietnam-China relation and also I doubt your definition of Pro-America in Vietnam.
Secondly , I said above and also replied in another’s question about Why Vietnamese people are Pro-America despite millions of Vietnamese dead caused by America during/related to the Vietnam war. I have read the survey conducted by some in Vietnam that they ask Vietnamese people’s opinions about America. I am pretty sure that Almost Vietnamese people tend to like America and also give a favorable opinion of America. If studying more about Vietnam culture, they are pragmatic, hospitable, and also ceremonious , especially to strangers or foreigners. For example: If You take a visit to a Vietnamese house when they are eating, they often invite you to join eating with them. But You should know that it may not be the real invitation if you are not invited before. Inviting you is the way to show their courteousness and hospitality. Moreover, generally, even though they could like or dislike you, but often they shall talk politely and respectfully with you. So, If you ask some questions about some countries which are not the current threat of Vietnam, Vietnamese people tend to speak some favorable words which you love to hear. But it does not mean that they are pro which countries you asked. The gap between “Pro-America” and reality is quite big. So, I do not say that the survey is wrong but I really doubt the highly pro-America you said.
Thirdly and Finally , the number of dead Vietnamese people you said in particular and the Vietnam-China border conflict 1979, in general, are only the small part in a few thousands of years of historical relation between Vietnam and China. Almost Vietnamese people dislike China but it does not mean that they hate or become anti-Chinese except China intends to invade Vietnam again. They could dislike you but it does not mean that they become anti-You and vice versa - They speak nice words to you but it does not mean that they become Pro-You.
P/s: I write answers based upon the gist and requirement of the question. Readers are requested to read it with neutrality and rational approach. I throughout this answer has no intention of showing any disrespect towards any group of people or country.