Why yes, the M-203 was fielded during the Vietnam War. This photo is from 1969-1971 timeframe, M-16A1/M-203.
NOTE: Quadrant sight is not installed on the carrying handle, the weapon only required the leaf-sight, located on the handguard, just before the M-16 front sight post.
US soldier armed with XM-177E1 Commando.
NOTE: called CAR-15 by soldiers (this was due to the first manuals fielded in 1967 with the weapon labeled CAR-15 from Colt.)
Also, no raised 'berm' around Magazine Release button.
Vietnam War circa 1967/1968
Size comparison of XM-16E1 rifle and XM-177E1 (CAR-15) Commando in UH-1C Huey Helo
NOTE: CAR-15 with two, taped 20-rd magazines inserted.
AN/PRC-77 Man Portable FM Radio behind CAR-15