Comment by acdha

Again, we don't have hard proof. The theory is while Nixon had a channel to the South Vietnamese he hadn't used it effectively but stepped it up when Kissinger tipped him off that the peace talks might be moving again:

> According to Haldeman’s notes, Kissinger alerted the Nixon campaign in late September, and again in early October, that something was up. Johnson was willing to halt the U.S. bombing of the North, and with the Soviets applying pressure on Hanoi to meet certain American conditions, the odds were never better for an early settlement of the conflict, which had already claimed 30,000 American lives and torn America apart.

https://www.politico.com/magazine/story/2017/08/06/nixon-vie...

No, that doesn't mean that the responsibility is entirely his – Nixon in particular shouldn't have been ignored after impeachment – but it does raise the question of how many people would have lived had Kissinger not bought his way into the National Security Adviser. He must, after all, have delivered something of value to have not only switched sides (he'd previously been affiliated with Nixon's opponents) but done so moving into a prestigious position without any prior government service.