Communism and free-market Capitalism are at the opposite ends of the political spectrum from each other. The former promotes state regulated and run commerce and industry, no room for private enterprise, everything goes to and is redistributed by the state. Capitalism promotes free trading and private ownership of means of capital. Capitalism is often associated with (it's clear to establish here that this is not always the case) democratic political procedures and establishments. It's also worth noting that Communism is heavily anti-religion and atheistic, the USA is a country founded upon and influenced by Christian ideologies and principles.

To narrow the scope a bit, after WW2, foreign policy in the US drastically changed, as it briefly did after WW1. It was realized that the US could no longer hold isolationist views of the world and that conflicts thousands of miles away could come back to affect them. Combine this with rapidly developing, industrializing, communist nations such as the USSR and China who were encroaching on lands occupied by or heavily backed by the US (South Korea, Western Germany/Europe) and you begin to see why the US were so concerned.

For Vietnam in particular, this ties into something called the Domino Theory. This theorized that the spread of communism and other 'extreme' ideologies was so prevalent due to it's spread through neighbouring countries, like dominoes falling against each other. These countries were generally found to be poorer nations, suffering from poverty, political corruption and/or ineptitude, recent shock and destruction from wars (worth noting that not all of these had to be in place i.e. pre-nazi germany). Vietnam was more or less suffering from all of these before US involvement during the colonial disputes Ho Chi Minh and the Vietminh had during the First Indochina War. This First Indochina war led to the split between the Communist North and the democratic, capitalist, Nepotistic South.

In the eyes of the US, the communist North represented a threat through the domino theory. If Communism spread to South Vietnam and surrounding Cambodia and Laos, then it could feasibly spread onto Thailand, Myanmar and potentially the Philippines, a direct ally of the US. From here it could either head further south toward Australia, or more importantly, head toward Hawaii and the US mainland. Conversely it could spread toward India & Pakistan and push through to Oilfields in Iran and the Caspian Sea, where American and British interests were already at stake.