After over 30 years, U.S. Army troops are returning to Vietnam. These training specialists will help to upgrade the combat capabilities of the Vietnamese forces. Because American troops are now the most combat experienced in the world, they are much in demand as trainers. Vietnam, on the other hand, has not seen combat since 1979, when there was a brief border war with China. There, the combat experienced Vietnamese beat up on the Chinese, who had not been in large scale combat (there had been some border skirmishes with Russia and India) since the 1950s.
The Vietnam live in fear of another Chinese invasion. Not because relations with China are currently frosty, they aren't, but because China and Vietnam have been feuding for about a thousand years. Vietnam was originally founded by Chinese fleeing the rule of the Chinese empire. Over the centuries, Chinese troops have occasionally entered Vietnam, trying to bring the area under Chinese rule. That potential, at least for many Vietnamese, remains, and thus the desire to have some American military trainers visit, and impart whatever combat wisdom they might have. The Chinese, as always, have the numerical advantage, so the Vietnamese want to gain a qualitative edge. Vietnam also wants to improve relations with the United States.