It Is Probably a Good Thing That This Plan Was Never Carried Out
An amphibious attack directly against North Vietnam had its supporters not only in the military, but also on the civilian side back in Washington, DC. However, while the plan stood a high chance of success against the North Vietnamese, it was deemed too dangerous because China would likely join the fray. At the time, only fifteen years had passed since the Korean War. In that conflict, US and allied forces led by General Douglas MacArthur had pursued routed North Koreans all the way to the Chinese border, based on the mistaken belief that China would not act. Unfortunately, the Chinese acted: they jumped in, and pushed MacArthur and his men all the way back to South Korea.
If China directly joined the war in response to a US invasion of North Vietnam, things could easily escalate from there into WWIII if the Soviets were dragged in. Worse, unlike Korean War days, America no longer held a decisive nuclear superiority: by the second half of the 1960s, the Soviets had thousands of nuclear warheads and the means to deliver them to targets in the US. American interests in Vietnam were simply not worth the risk, and the so the planned invasion of Hanoi-Haiphong was never carried out.