Political constraints stopped that from happening, not military ones. Bombing ships at the pier and blockading and mining harbors would have stopped many supplies before they even got started south.
THe decisions not to do so were political ones. Militarily, it made the utmost sense to destroy the materiel before it was disseminated.
In Korea as well, the objectives and 'limited goals' were under constraint by political forces. Had LeMay and Mac Arthur had their way, there would have been a decisive victory.
Both situations had one thing in common, They were not decided on the battlefield by soldiers so much as at the conference table by politicians. Make of that what you will, but I'd let the soldiers do their job.
In fact me mined the harbor for a period of 10 months and bombed both the city and rail lines going into Hanoi. It didn't work. They discovered for one thing the Vietnamese would move things by bicycle if deprived of fuel and for another that supplies were coming from thousands of dispersed cottage industry style factories.
What would have worked was cutting the Ho Chi Minh trail, but that couldn't be done with air power even though it was attempted the entire war. I know many people are fond of the "politicians wouldn't let us win" refrain but it simply wasn't so.
---h?f?ng?, city (1989 pop. 1,447,523), NE Vietnam, on a large branch of the Red River delta c.10 mi (20 km) from the Gulf of Tonkin. It is connected with the sea by a narrow access channel that requires continual dredging. A major port of Vietnam and one of the largest ports in SE Asia, Haiphong was developed (1874) by the French and became the chief naval base of French Indochina. A shipbuilding industry and cement, glass, porcelain, and textile works were established by the French. At the beginning of the French-Indochina War (Nov., 1946), French naval vessels shelled the city, killing c.6,000 Vietnamese. After the French departed and the state of North Vietnam was created (1954), the silted-up harbor was reconstructed with Chinese and Soviet aid, and the docks and shipbuilding yards were repaired and modernized. The old French cement plant was enlarged, and fish-canning, chemical-fertilizer, machine-tool, and additional textile industries were established. During the Vietnam War, Haiphong was severely bombed by the United States; the shipyards and the industrial section of the city were devastated, rail connections with Hanoi were disrupted, and thousands of homes were destroyed. The harbor was mined by U.S. naval planes in May, 1972, and effectively sealed until the mines were swept by U.S. forces after the cease-fire agreement in 1973. Reconstruction, while slow, was aided by the dismantling and relocation of many of the factories during the bombings; when returned to Haiphong, much of the machinery was able to function in ruined structures. Haiphong has been substantially rebuilt; a steel plant was built there in the mid-1990s.---
http://www.questia.com/library/encyclopedia/haiphong.jsp?l=H&p=1