After the festivities in Sydney concluded, she rendezvoused with Coral Sea and the newly formed Task Group 77.6 en route to Guam on 13 May.

As part of this operation, The Buchanan departed Hong Kong escorting the aircraft carrier Hancock with Southerland and Lyman K. Swenson on 2 November.

The coup had ended with assassination of Diệm, his brother Ngȏ Ðình Nhu, and Col. Hồ Tần Quyền, the commander of the Vietnamese navy.

With reassurance from the new leadership under General Dương Văn Minh that order had been restored and alliances unchanged, the Navy reduced its presence off of South Vietnam.

Several days after emerging from overhaul, she rejoined Destroyer Squadron 15 to participate in fleet exercise "Union Square" (28 September – 1 October 1964).

On 1 February Buchanan began her tour of duty patrolling, providing escort and plane guard services in the South China Sea.

As the situation worsened in Southeast Asia, President Lyndon B. Johnson began increasing the number of American troops in South Vietnam.

In mid-September, the destroyer entered Long Beach for overhaul (16 September–11 November) then spent the rest of the year conducting local operations.

For her time in the South China Sea, Buchanan was awarded the Armed Services Expeditionary Forces Medal for supporting air strike operations over Vietnam.

She broke from her combat operations to serve as the flagship of the United States Seventh Fleet during a port visit to Bangkok, Thailand, on 25–29 July 1966.

She also provided naval gunfire support (NGFS) to ground troops fighting in South Vietnam, most notably during the first phase of the Tet Offensive and the Battle of Huế (30 January – 3 March 1968).

During this deployment, the destroyer once again served as a plane guard for Ticonderoga in the Gulf of Tonkin (8–24 May) and provided NGFS for troops in the I and II Corps Tactical Zones.

Once back at her home port on 18 September, Buchanan underwent a survey and inspection (24–26 November), ending the year working with destroyer tender Samuel Gompers.

They were given slight respites with several visits to Subic Bay, the longest being for upkeep (21 August–1 September, 1–10 October) before returning to combat duty.

On 1 April, Buchanan, in concert with guided missile destroyer Joseph Strauss provided gunnery support to cover the withdrawal of the South Vietnamese 57th Regiment from Firebase Gio Linh, near Ðông Hà.

Ripley sighted four enemy amphibious light tanks, Soviet-made PT-76, moving along the Cam Lo riverbank east of Ðông Hà.

Ripley sent a letter to the captain of the Buchanan, CDR William James Thearle, on 8 September 1972 describing the NGF support received during the operation.

Lt. Robert S Nemmers, Supply Corps, SA Vincent G. Guerrero, SD3 Primicitis V. Beltran, EM Frederick J. Shortreed Jr., GM3 Danny K. Hammond, and CS3 Frank S. Musiol were wounded.

For her remaining time in Southeast Asia, Buchanan served as a picket ship and plane guard supporting the air strikes associated with Operation Linebacker (2 May–23 October 1972).

During this period, her 127 mm guns and ASROC launchers were removed along with her aging Mark X Identification Friend or Foe, SAMID, and other communications equipment.

On the first day of 1976, Buchanan moved back to Long Beach for some final touches to her overhaul then remained close to home conducting routine operations for the entirety of 1976.

Buchanan departed Guam on 16 May and was able to rejoin her task force in the South China Sea for "MAULEX 3–77", a joint American-Philippine amphibious exercise.

Buchanan took part in one last multinational exercise, "SEA SIAM VI" (19–21 July) before tying up at Subic Bay for maintenance and repairs (29 July–21 August 1977).

Despite her busy schedule, Buchanan was afforded some breaks to visit Kaohsiung (27–28 May), Pattaya Beach, Thailand (14–18 July), Fukuoka, Japan (3–6 October 1977).

While en route to Pearl Harbor (2–4 March), a companion guided missile frigate Brooke, suffered a failure to her main steam condenser.

Over the next several months, Buchanan participated in multiple exercises off the Korean Peninsula (7–10 May), Okinawa (16–23 July) and in the Tasman Sea (18–23 August 1979).

When she was in port at Pusan (5–8 December), sailors from Buchanan and Holt delivered food and clothing to a local orphanage as part of Operation Handclasp.

Wrapping up her deployment, she made her way home to San Diego (9–17 September) where she immediately entered a period of upkeep and giving her crew leave to be with their families.

After spending the first five months of 1984 operating out of San Diego, Buchanan moved to Hawaiian waters to take part in Rim of the Pacific (RIMPAC) '84, anti-submarine warfare exercises and missile tests.

In response to threat, the U.S. executed Operation Earnest Will to provide protection to the tankers and ensure that commercial shipping lanes remained open.