Then you've forgotten. When Jimmy Carter was President, people were questioning whether the West would survive.
As to Vietnam, the war had been won in 1972. It was then lost in 1974, when the Peace Democrats cutoff military supplies to South Vietnam.
Finally, as to the importance of ideology, here is a thought experiment for you. If Reagan hadn't believed the Soviet Union was an evil empire and hadn't believed that the guiding principle for the Cold War should be "We Win, They Lose . . .", would the Soviet Union still have imploded.
Put another way, it's pretty clear that Reagan's beliefs dictated his actions, not the other way around.
Ideas are everything, competence less so.
Then we obviously don’t see things the same.