Timely troop movements, troop support, supplies, and strategic air stikes.
The countries that were actually "physically" involved, and were bombed or had US aircraft crash into them, whether they were "officially" in the war or not, were: Lao's, Cambodia, North Vietnam and South Vietnam.
No one
There were two wars: The air war over North Vietnam and the ground war in South Vietnam. The US Air Force and US Navy (aircraft carriers) fought the war in the North. The US Army and Marine Corps fought the war in the South. USMC jets supported their marines in the south. The US Army and US Marine aviation units, for the most part, stayed out of North Vietnam.
The Vietnam War escalated from a Viet Cong guerrilla war to a war involving regular Army units of the North Vietnamese Army; including jet aircraft combat between US Air Force/US Navy jet fighter planes against North Vietnamese Air Force fighter aircraft.
The US offered to help south Vietnam in 1961 to help Vietnam preserve its independence from the North. It was during the Cold War, and the US was fighting Communist Russia across the world.
During the Vietnam War, the US Army had the largest helicopter force in the United States.
US was a primary combatant.
The US asked for their help.
RVN (Republic of South Vietnam); which no longer exists.
Yes indeed; Skyhawks played key roles in the Vietnam War, the Yom Kippur War, and the Falklands War. They were a superb low level strike aircraft and have used by many air forces around the world. In Vietnam they were used from US Navy carriers.
Because the war was to help a different country
Guerrilla warfare was largely used by the Viet Cong and not by the US Combatants in Vietnam. To answer your question, no.