Why was Vietnam important in the domino theory?

Domino theory is important because if one country fell that meant that others would fall in the aftershock. America could not allow this because they were trying to stop communism not have other places pick it up. So the Lyndon Johnson who was the president then that he would help the south Vietnam to defeat the north so they don't fall into communism but it all went wrong with many people getting hurt and killed in the south they decided that when election came by so they could choose between communism or capitalism that they would choose communism as that seemed the better choose. but what they did not know that there would be bombings that would stop the election so they could not choose. So if the domino theory was correct then when one powerful country fell into communism that all the country's it has ties too would fall to.