First, note that other nation's in the region did fall to Communism after the United States withdrew from Vietnam. After years of violence, Communists took power in Cambodia (the "Khmer Rogue regime") and Laos in 1975. Just as many critics had warned, after the United States withdrew from Vietnam in 1973, other nations in Southeast Asia quickly fell to Communist forces.

So why, then, do historians now look back at the "domino" theory originally articulated by Eisenhower as a "fundamentally flawed" concept? After all, the idea was that when one nation fell to Communists control, its neighbors were likely to follow. Didn't the Communist victory in the Vietnam War prove that?

No, it did not. In fact the opposite occurred. You see, the fundamental flaw in the reasoning was that Eisenhower and others that followed him mistook Communism for a monolithic power (i.e, a large, powerful, and intractably indivisible and uniform movement) that expanded Moscow's dominion with every nation it overtook. In Turkey, Greece, Korea, Vietnam, and all those nations finally finding their freedom from European control, American policymakers saw Communists as loyal soldiers for the Communists movement as a whole. Americans feared--and arguably they had reason to, at the time--that a great Communist power would soon cover so much of the globe that it would infiltrate Western Europe, the Americas, and eventually the United States.

It turned out, however, that "Communists" around the globe had different motives for adopting Communism, and that, rather than a monolithic force, they represented a diverse group whose values and interests would come into conflict with one another, especially as the movement grew on a global scale. The Bolsheviks in 1917 wanted to overthrow the Tsarist system in Russia; Kim Il Sung wanted to reunite Korea, a nation that had been first annexed by Japan and then divided by the US and the USSR after the Second World War, under his control; Ho Chi Minh similarly wanted to finally expel Western "imperialists" from Vietnam (having defeated the French by the mid-1950s only to find his nation divided like Korea). Ironically, it was the Soviet-American obsession with "bipolarity" that, for a time, pushed the politically active to chose one side or another. Ho Chi Minh chose Communism because he knew that the Communist movement (led then by Moscow and Beijing) would help him in his fight against the Americans. Remember, Ho Chi Minh, as a much younger man, traveled to the Paris Peace Conference after World War I with the hope of securing Woodrow Wilson's support for Vietnamese "self-determination" (Vietnam being at the time a part of French Indochina), but Wilson refused to meet with him. Vietnam's struggle for independence could have just as easily been fought under the flag of "democracy" as it later was under the flag of "communism" in the 1960s.

The specter of monolithic Communism splintered with time. Even as the Vietnam War peaked, the "Sino-Soviet" split had begun between the Soviet Union and China, especially after Stalin's death and the de-Stalinization that began under Khrushchev in 1956 (Nixon took full advantage of this in the 1970s with his so-called "triangular diplomacy," where he played off of mutual Chinese and Soviet fears to improve relations with both). And even in Southeast Asia, where the United States fought and lost a war to prevent the toppling of "dominoes," relations between Cambodia and Vietnam collapsed to the point that Vietnam invaded and occupied Cambodia in 1978. Without the common American enemy to fight anymore, nationalist impulses (the Cambodians feared that the Vietnamese would try to create a regional Communist federation dominated by Hanoi) took precedent, and the "Communist monolith" proved to be nothing but a figment of the American imagination.

Some primary and secondary sources on this topic:

For the original and clearest articulation of monolithic Communism, the threat it posed to the United States, and the need to "contain" Communism, see George Keenan's (a lower member of the American embassy in Moscow at the time) "Long Telegram," written in 1946 and often cited as the foundation of American perceptions of the Cold War. Note specifically how on page 703 (link starts on page 700) Keenan warns that while Communists may appear to be working independently, "they are in reality working closely together as an underground operating directorate of world communism, a concealed Comintern tightly coordinated and directed by Moscow." Follow up with "Article X" if you are interested in Keenan's continued influence (this was a more formal article he wrote in defense of the proposals he made in the Long Telegram).

Eisenhower's domino speech, which specifically addressed the need to maintain influence in Vietnam after the French withdrawal, is the most famous articulation of Keenan's "containment" argument.

Domino theory was not created in a vacuum, but was an extension of the American perceptions of Munich. Chamberlain gave Hitler Czechoslovakia, which only feed his aggression and allowed Fascism to spread. For a great analysis of the links between historical analogy, dominoes, and foreign policy:

Yuen Foong Khong's Analogies at War: Korea, Munich, Dien Bien Phu, and the Vietnam Decisions of 1965 (University of North Carolina Press, 1991).

For an analysis of the American construction of the Communist "monolith" and the lessons that we can learn from this mistake:

Marc Selverstone's Constructing the Monolith: The United States, Great Britain, and International Communism (Harvard University Press, 2009).