Cohen, who arrived from Hong Kong, is expected to hold talks with Vietnamese leaders in Hanoi on developing modest military ties, perhaps in non-combat areas such as demining, humanitarian aid, flood relief and tropical medicine.
A focus of Cohen's trip will also be joint efforts in accounting for some 2,000 American servicemen still listed as missing in action from the Vietnam War. Cohen will visit an MIA paddy field site on the outskirts of Hanoi later on Monday.
Cohen has said he would not bring to Hanoi any apologies over the war, in which the communist North defeated the U.S.-backed South Vietnam. He has said the American and Vietnamese militaries would move slowly in establishing ties.
His visit comes amid two months of celebrations in Vietnam to commemorate the 25th anniversary of the end of the Vietnam War on April 30, a conflict in which an estimated and more than 58,000 American troops died.
Cohen's Vietnam visit is part of a major Asian trip. After Vietnam he will go to Japan and South Korea.