Celebrants atop the falling Berlin Wall on November 9, 1989. Gorbachev saw Soviet communism's collapse as a new beginning, but many U.S. policymakers hailed it as the ultimate vindication.
Your general interpretation is fine.
'Vindication' is defined similarly in the dictionary.
The quoted passage does not give details on whether vindication refers to what had been said or done or U.S. policymakers. If we follow your sentence construction,
the fact of proving that what US policymakers said or did was right after other people thought they were wrong
then it refers to the former.
Other than giving a few suggestions, I have changed the pronoun near the end of the sentence to singular to maintain consistency; the pronoun has antecedent what US policymakers had said or done rather than US policymakers.
"the fact of proving that what US policymakers [had] said or done was right after other people [had] thought [it was] wrong"