John Kerry didn't go to Paris "secretly" - he was on his Honeymoon.
From the blog on Democrats.org:
While he was there, he was approached by the negotiating teams from North Vietnam (Who were *already* in place with representives fromboth the U.S., North Vietnam and South Vietnam). He took the opportunity to once again fight for American soldiers, specificall your POWs.
He met with Madamn Mihn and was told that no American POWs would be released unless America unilaterally withdrew from Vietnam. Further, he reported the contact to Congress upon his return:
"I have been to Paris. I have talked with both delegations at the peace talks, that is to say the Democratic Republic of Vietnam andthe Provisional Revolutionary Government and of all eight of MadamBinh's points it has been stated time and time again, and was stated by Senator Vance Hartke when he returned from Paris, and it has been stated by many other officials of this Government, if the United States were to set a date for withdrawal the prisoners of war would be returned. I think this negates very clearly the argument of the President that we have to maintain a presence in Vietnam, to use as a negotiating block for the return of those prisoners.
-John Kerry, April 1971.