This is answered in this thread to some degree: http://www.reddit.com/r/AskHistorians/comments/236fa8/why_didnt_the_us_just_roll_over_north_vietnam_and/

Basically it's one of the biggest reasons why the US didn't mount a full scale ground invasion of North Vietnam and settle the NVA's hash once and for all. We bombed the hell out of the country, but our military high command was worried that a ground invasion would illicit a massive Chinese intervention just like what had happened when we attempted to take North Korea after pushing them out of the South.

We wanted a a situation in Vietnam like what occurred after the Korean War ended; the two sides divided and secured. That of course did not happen.