Kerry also went to Paris to advise the North Vietnamese and Viet Cong negotiators during the Paris Peace Talks.
The above is an account of that from2014 & updated in 2015, but the claim is that he met with Madame Nguyen Thi Binh, then-foreign minister of the Provisional Revolutionary Government. However, the claim is that he didn't negotiate anything with anyone. Not sure I believe what he says though.
But you are correct he was over there at that time, and he was a Senator at the time also.