Korea was pretty traumatizing OTL, one of the reasons the US was so politically circumspect in Vietnam was that it was afraid of another Korea
The Korean Peninsula is ~200 miles wide or less, the South Vietnamese border was over 700 miles long, the US could maintain a continuous line across the Peninsula it could not do in Nam and stop flow of supplies to any guerillas in the South. The only way to make the lines wide enough the US can't cover them to stop supplies going to guerillas, is for the US to occupy the whole Korean Peninsula. The issue there is that means the Communists have lost a lot of cred, and are relying on bases within China and the USSR, making them look like foreign puppets, plus the US can just withdraw the 39-40 parallel and it has a line it can control, and SK still owns Pyongyang and half of North Korea, that's a victory
This assumes that the war is unconventional. Conventional it is hard for it to end up too much worse than OTL, Armistice around the 38th parallel was the goal after Chinese intervention. The US and allies had enough conventional superiority they could force an armistice if the Chinese would not accept one on reasonable terms