There is more evidence for it today than there was at the time of Hitchens' death. In 2016, notes from HR Haldeman - Nixon's future Chief of Staff, famously brought down by Watergate - were published that reference then-candidate Nixon's efforts to sabotage peace talks. And in 2013, some White House LBJ tapes were declassified in which LBJ is recorded talking about Nixon's sabotage - uncovered by US intelligence - to aides. On the recording in the above link, LBJ can be heard saying:

"We have found that our friend, the Republican nominee, our California friend, has been playing on the outskirts with our enemies and our friends both, he has been doing it through rather subterranean sources. Mrs Chennault is warning the South Vietnamese not to get pulled into this Johnson move."

Chennault was a senior Nixon campaign adviser. The North Vietnamese had demanded a cessation of bombing as a pre-condition for peace talks, and LBJ was trying to get the South Vietnamese to go along with a planned US halt in order to allow peace talks to happen. If the South Vietnamese aren't participating in the ceasefire, then there won't be peace talks, and there will be no peace agreement found. Because Nixon convinced the South Vietnamese not to go along with LBJ's ceasefire, he prevented the peace talks from happening. Nixon was running on a campaign of de-escalation and US withdrawal; should a peace plan be struck before he became President, his major foreign policy idea would have been made irrelevant. Nixon seems to have derailed peace talks primarily to help himself win the Presidency.

LBJ ultimately decided to do nothing about it, because he didn't want to reveal that the FBI was listening to the conversations of the South Vietnamese ambassador. His advisers also feared what the publication of that treason would do to American trust in the Presidency. One adviser can be heard on another tape saying:

"...Some elements of the story are so shocking in their nature that I’m wondering whether it would be good for the country to disclose the story and then possibly have [Nixon] elected. It could cast his whole administration under such doubt that I think it would be inimical to our country’s best interests."

Nixon always denied he did anything, of course. But the evidence seems to show that he did.

Sources: NYTimes, BBC, already linked. The times article linked has the primary source info - Haldeman's notes - and the BBC article provides the general story, but also has an audio recording of Johnson talking about Nixon.