Good overview of the story is here:
LBJ wouldn't divulge how he learned about Nixon's torpedoing the peace talks, because:
1) He learned about it with secret FBI Wiretapes on the South Vietnamese Ambassador's phone.
2) He had already said he was not running for re-election. It affected Humphrey, not LBJ.
3) Nixon could have spilled the beans about LBJ's complicity in the JFK Assassination in Dallas.
Dirty, dirty business. The 1960's was a very dirty political era in all shapes and forms.