OK, here is the photo out of a book I have on Vietnam. The caprion states this was taken in Pleiku. They are certianly TOW systems. The troop sitting on the hood of the 2nd 151 appears to have a 1st Cav shoulder patch
I found the following info on the web about TOW ground system usage in Vietnam:
30 April 1972 DA ordered the deployment of the ground-based TOW system with instructors to train U.S. and South Vietnamese crews to operate the weapon.
May 1972 An 82nd Airborne antitank task force was airlifted to Vietnam with 24 jeep-mounted launchers, 500 missiles, and two ¾-ton trucks from the maintenance contact team. The task force consisted of a 48-man crew plus a maintenance contact team of 10 personnel from the 763rd Ordnance Company. In Vietnam, units of the task force were attached to the 3rd Brigade, 1st Cavalry Division (CD), whose personnel were trained to operate the TOW weapon system
10 May 1972 TOW training for South Vietnamese Marine Corps personnel began on this date and continued through 22 July. They fired a total of 163 TOW missiles. During the course of this training program, the first ground-based TOW was fired in actual combat.
21 May 1972 By this date, 28 missiles had been expended in training personnel of the 82nd Airborne Division and the 3rd Brigade/1st CD on the ground-based TOW weapon system. Gunners of the 82nd Airborne task force fired 12 training rounds against an artillery bunker at a range of about 2800 meters and scored 12 direct hits. Gunners of the 3rd Brigade/1st CD fired 16 training rounds, with 1 missile malfunction, 9 target hits, and 6 misses due to poor lighting conditions.
22 May 1972 The first tank kill by the ground-based TOW deployed with the South Vietnamese Marine Corps occurred when an NVA combined tank-infantry force with 9 tanks and about 200 troops attacked the 369th Brigade command post (CP). When the battle ended 2 hours later, all 9 tanks had been destroyed and 117 enemy were confirmed dead.
26 May 1972 The 82nd Airborne Division task force, previously moved from Bien Hoa to Pleiku, was rushed to positions around Kontum to help counter the NVA tank assault launched on this date. PFC Angel Figueroa scored the division's first tank kill with the ground-based TOW during the main battle for Kontum that raged until 31 May. About a week later, the 48-man task force turned over their TOW equipment to the 3rd Brigade/1st CD, then returned to the United States.
25 June 1972 A limited number of U.S. ground-based TOW missile systems saw combat in Vietnam. These systems destroyed 12 tanks, including 9 in a single action northwest of Hue on this date.
19 August 1972 By this time, a total of 23 ground-based TOW missiles had been fired in combat engagements, destroying 11 tanks and 6 bunkers.
No date is listed in the article for the withdrawal of the 1st Cav TOW units.