The United States entered the Vietnam conflict to prevent Southeast Asian countries from falling to the Communists.
Two separate countries: South Vietnam was officially called RVN (Republic of South Vietnam). When US servicemen landed in country, they were greeted with "Welcome to the Republic of South Vietnam" (enjoy your stay).
Vietnam was split into two countries during the vietnam war, the north was ruled by Ho Chi Minh and the South by Ngo Diem.
Only one nation won the Viet War; North Vietnam. Interesting question, which two nations did you have in mind?
Laos and Cambodia were both drawn into the Vietnam War because of atrocities the people of these two countries suffered under the hands of North Vietnam. South Vietnam and other countries were trying to keep the North Vietnamese army from spreading Communism in these countries.
North Vietnam and South Vietnam. Also, to name just a few: the USA, China, Russia, South Korea, Australia, France, and England. A great many countries fought in the Vietnam War from 1945 to 1975.
After the war, there was a reunification of Vietnam. So the answer is "One country".
Laos and Cambodia.
North Vietnam brought the break away democratic south back under communist rule and re-unified the country.
Vietnam , America , and Russia
Primary combatants were US & North Vietnam; especially the air war over the North.
North Korea & North Vietnam.