the fighting would just "pop up" almost anywhere in the country at any given time
Da Nang or Chu Lai could get a rocket attack,,,,,then a day later after that was defeated,,,,,,an outpost close to the Ho Chi Min trail would get hit,,,or farther south
if you think of how Montana would be attacked by 1,000 people while being defended by 100,000 troops,,,,,,that was about how the attempt was made to end the civil war in VietNam (sort of a reverse of the movie "Red Dawn")
the fall of Saigon was different than most of the war before that time period,,,,,that was a traditional "large force advances on a goal" type of fight,,,,,,without the US participating in the attempt to halt it