Did the US successfully protect south Vietnam from communism?

Yes, the United States and its allies did technically "contain" communism, but the fall of the Soviet Union was less of the United States' work and more the fault of the Soviets themselves. The Kremlin's attempted cover-up of the Chernobyl nuclear disaster and other missteps in their economic policy cost the country billions, and eventually led to the demise of the once powerful nation and the introduction of the Commonwealth of Independent States in 1991.