Our stance towards communism in Vietnam was the result of what was called the Domino Theory. It was the belief that if a few nations fell to Communism, then many more nations would fall to communism in term.

Apart from the totalitarian oppressiveness of 20th century communist regimes; the notion of public ownership of, well, everything, is sort of antithetical to capitalism, which is at the core of a lot of so-called American values. Capitalism exalts the individual and (theoretically) is meritocratic, meaning people succeed from their own talents and work. Communism shuns the notion of individual success except as a productive means to assist the common good.

Communism as a form of government has more to do with the rhetoric and ideals than the actuality. No government has really succeeded at Marx's communism, but the term generally has to do with an extremist socialism, where the government, as the nominal representative of the people, controls everything. This is very statist, and can often be totalitarian.

The reason these communist countries are more scary than they allegedly would be if they were democracies has to do with a theory that democracies don't go to war against each other. The basis being, maybe the leader of government A has some reason to go to war with government B, like it would help expand the borders of country A. In totalitarian communism (which is the only communism we've really seen), A can just declare war on B, and order his soldiers into battle for the greater glory of whatever. The notion is that in a free democratic society, it will never be in the majority interest of one democratic population to pursue war against the other, because there would be no important gains from doing so, and the majority of the population doesn't like to go to war, especially if it puts relatives in danger. You can't paint a democratic nation as an evil spectre so easily.