Looking at the map, it seems to me that one strategy to consider is hold in the south and then attack in the north with a Battle of Inchon-type invasion rather than get bogged down in the south. Was this considered?
Because if they did that it would have escalated the war like what occurred in Korea. It was well within American capacity to launch an invasion of North Vietnam even by land but if they did that its highly likely China and even Russia would have countered. This is something they wanted to avoid at all costs. America's intention in Vietnam was therefore not to destroy the North but simply make sure that South Vietnam remained separate and not fall to the communists. This is why the war lasted as long as it did as America was unable to escalate the conflict to bring about the decisive end. All they could do was defend the South and make sure the government remained in power.