The US military was unable to defeat the conventional Vietnamese military. It was very much a different war to Afghanistan. The US military failed to degrade Vietnamese logistics and force generation capability to prevent North Vietnam from invading and occupying South Vietnam. There were guerilla tactics, but the NVA was very much an industrialized and mechanized military, despite failed US attempts to degrade that.
> What they can't do is force people at gunpoint to adopt American and/or modern or western values
The Soviets didn't have to do it. They were acting in support of an existing Afghan state which was fighting a war against Pakistan and US funded Mujahideen which wanted to overthrow them. Plenty of people in Afghanistan were open to modern values. The issue with the Soviet invasion of Afghanistan is that the USSR fell, and the issue with the American invasion is that there was no pre-existing state and that the US didn't have a compelling value proposition for Afghans to die in defence of after they pulled out.
Afghanistan is not some kind of bizarre land where everyone hates the concept of modernity. The Americans just didn't figure out how to organize an Afghan state, and didn't figure out how to win militarily either. The Soviets figured out the former, but they collapsed. Though their state survived the Soviet pullout from Afghanistan until the collapse of the USSR and the end of economic aid.