Was america justified going into vietnam? what if the communist countries invaded a country to contain the spread of democracy, would this be justified? what is the difference between the two situations?

The socialist nations did precisely that. North Korea attacked South Korea, North Vietnam attacked South Vietnam, The Soviet Union attacked Czechoslovakia, Poland, Easter Germany, and Hungary amid the 1950's and 1960's to dispense with alleged "majority rules system developments.' So you see it works both ways.