The South.
The South Vietnamese
America was defending Vietnam.
North Vietnam invaded South Vietnam. Russia and China and to a smaller extent Cuba came alongside North Vietnam to help their cause. The United States, Canada, Australia, Philippines, and Korea aided South Vietnam in defending their sovereignty.
Vietnamization was part of President Nixon's plan for "Peace with Honor" by training, equipping, and then turning the full responsibility of defending South Vietnam to South Vietnam's military units.
Air war over North Vietnam. Ground war in South Vietnam; Riverine war in South Vietnam.
The answer is yes, South Korea was in the Vietnam War!
The United States lost the will to continue fighting. The Vietnamese, who were defending their own country, were willing to fight forever. The North Vietnamese saw no distinction between North and South Vietnam. Ed Vietnam War Veteran 1968-69
Air war-North Vietnam Ground war-South Vietnam
North Vietnam started the war against South Vietnam; the US went to the aid of the South.
North Vietnam. Actually, the credit should go to the NVA (North Vietnamese Army) they did all the work. The NVAF (North Vietnamese Air Force) did a lot of work too; but they were DEFENDING North Vietnam. Not invading South Vietnam.
At the end of the Vietnam War, Congress refused to support South Vietnam any longer.