Thai and Vietnamese officials pledged to significantly increase bilateral trade and integrate supply chains during a recent diplomatic visit.

It was a gesture that was equal parts diplomacy and theatre: Thai Prime Minister Anutin Charnvirakul coaxing a melody out of a traditional Vietnamese t’rung xylophone at a Hanoi state banquet on Monday. The real music, however, had been made in the meeting rooms. Two days of talks between Anutin and his Vietnamese hosts produced a pledge to nearly double bilateral trade to US$25 billion within four years – and eventually to double it again. Supply chains would be stitched together across...