Throughout the war there were two very clear reactions from the American population to the country's involvement in Vietnam; the pro-war (supporters of the war) and the anti-war (non-supporters of the war). Thousands of people "changed sides" from one to the other as the conflict progressed. The government paid little attention to them however at the end of the war the main factor of the US's withdrawal and loss was the lack of support from the American people, as compared to the vast majority of people being pro-war in the beginning and turning anti-war nearer the end.
At the beginning of the War the majority of the US population were pro-war. There were many reasons for taking this view. Most Americans would listen to and respect their presidents and the stirring speech by President Kennedy led the population to trust him, his government and their beliefs. Media and television were becoming a popular medium for winning support; speeches could be televised and seen by all and news of the war's progress could be broadcast, but also the reality of war could be brought into the people's homes, and at the beginning of the war the reality looked good for the US.
Technologically the US was far more advanced with its development of napalm and anti-personnel bombs and chemical and biological weapons such as Agent Orange and Agent Blue. With the knowledge that America had the most advanced military in the world the supporters believed that the staggering firepower would easily defeat the Vietcong.
With reguards to the Domino Theory, the fact that it was devised by Kennedy's government and the fear of the spread of communism at the time (in China and Cuba), the theory would certainly seem plausible and possible. Left alone the spread of communism could be exponential...