1) Not true. It was a stalemate at best. When the Paris accords were signed, South Vietnam controlled about 80 per cent of its area. North Vietnam was under no US control.
2) Afghanistan was a mess. The Americans should have learned from the Russians before them, and the British before the Russians.
In a nuclear war, there would be no winners. The US has more warheads, but Europe has its own nuclear arsenal. In a conventional war, I'm sure the US would think it would win easily. That is probably what Russia thought in the Ukraine. Most of the big European armies - the UK, Germany and France, are much better trained, no conscripts, and have enough modern technology to give the US a hard time and not make it worth their while. Sure, they'll be the usual braggadocio from the US right, but they were the ones puffing their chests during Vietnam and Afghanistan too. Europe is no Iraq under Saddam. Trump knows that. Bannon and Miller do too, but they don't care because like the Don they are fellow psychopaths (BTW, that last word, that isn't hyperbole or rhetoric - all three meet the clinical defintion - easily).