I wish I knew for sure.
My guesses are –
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something to do with repatriating the remains of the 500+ POWs left behind, who were probably all deceased by 1986 (executed if they hadn't died of natural causes, or torture, malnutirition, etc.)
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financial fuckery having to do with Japanese gold (the Japs left a lot of their stolen gold in Vietnam and Cambodia in 1945. was a big reason France decided to re-colonize VN and fight the Viet Minh)
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negotiating U.S. legal support for Vietnam's claims in the Paracels and Spratlys.