A win for the US population's ability to make themselves heard, a defeat for the Pentagon.
The "Pentagon" is a tool the politicians use. Blaming the "Pentagon" for Vietnam is like blaming the gun for murder.
We won every major battle, had a very good kill:death ratio despite the sh**tty conditions (the military was designed, at the point the war started, to fight on the open fields of Europe, not the jungles of southeast Asia) and left on our own terms. A win? well no, obviously not as we didn't reach our goals, but I've got a hard time calling it a loss.
It doesn't matter if the US had tactical success. Wars are military means to political goals, and North Vietnam achieved its political goal (unification of Vietnam under Hanoi) while the US failed to achieve its goal (survival of the Saigon government).