The Bombing stopped in January 1973 when North Vietnam, the Viet Cong, South Vietnam, and the United States signed the Paris Peace Accord (not a bipartisan law). In the Accord, North Vietnam and the Viet Cong agreed to stop all aggression if we would stop bombing them to oblivion. The Accord also contained a provision that the United States would continue to arm the South Vietnamese and replace their weapons - bullet for bullet, aircraft for aircraft, etc... so long as they were using them in defense against aggression from the North.
In January 1975, a Democrat Congress defunded our commitment, under the Accord, and North Vietnam started invading South Vietnam again.