The only way to win is to convince the North Vietnamese Politburo that victory cannot be won in the short term, and that the costs of the war are too great for the benefits. While the North Vietnamese would intend to restart the war at a later point, in hindsight this would probably be the end of large term hostilities.
Two things need to happen.
First, the North Vietnamese need to think the US has not lost its willingness to keep fighting the war. That means a much better spin, reaction from the American news media. How this can be done realistically is beyond me - I think it would need a much earlier POD.
Second, the North Vietnamese would need to see not just a defeat for the Viet Cong, but that the price paid in the north had reach a point they needed to capitulate. That means either a heavier, more successful bombing campaign, or an incursion into North Vietnam - even if it is temporary and not with occupation. Basically a punitive expedition to destroy the starting point of the Ho Chi Minh Trail and ravage the staging areas of the conventional forces of the NVA. If the North Vietnamese no longer thought their country was a safe area, it changes the dynamics of supporting a war in the south. It would also signal a change in American resolve.
This would be considered to be a huge escalation. At the time, it would probably be seen as terribly provocative and risking Chinese intervention. In hindsight, we know this would do no such thing. However, to do so would probably require LBJ to decide he can't win re-election, but that if he can turn the screws on North Vietnam, he might leave a better military situation to his successor.