In February 1970 Kissinger met secretly with a Communist politician who served as an advisor to the North, who advised that any peace settlement must encompass both a military and political solution, a coalition government with Vietcong representatives, and a replacement of the Saigon regime.
This was unrealistic, the USA couldn't betray the South on such an extreme scale.
The North began demanding the removal of Thieu in the South, but the South would collapse into chaos and see this as a US betrayal, but in October 1972 the North changed their position regarding this to allow for the end of the war.