Here's the problem with that.
North Vietnam: does not want LBJ and Thieu to get an agreement that guarantees the survival of Thieu's government, believing that based on RFK's own record, he will allow a coalition to form instead.
LBJ: is a lame-duck, with all the ineffectiveness that means.
RFK: will not stake as much on Thieu personally, but he will probably come to realize that Thieu, while far from perfect, is the best option available. He's nothing if not a shrewd negotiator and a realist.
Thieu: wants GVN to be recognized as legitimate.
Possible terms: withdrawal to the 38th parallel, a demilitarized zone, and an informal non-aggression pact. RFK will not want permanent bases in RVN. That means shipping US frontline equipment. My usual list is as follows: Phantoms, Skyhawks, Herk tankers, ECM equipment and refuelling probes. Thieu was already going through the process of removing as much dead wood from the officer corps as possible, since the RCC and of course Ky (IMO a right-wing Chavez), were hardly enthusiastic.