How did the US Marines prevent being overrun by the NVA at Khe Sahn
during the Vietnam War?

Well "Duh!"
They were US Marines. Fanatical devotion to their
fellow Marines, their honor, and their country!

or for another view from Quora:

Al Clark
Former Retired, Former Sgt USMCMarch 31

How did the US Marines prevent being overrun by the NVA at Khe Sahn
during the Vietnam War? Was it massive airpower and determined defense
by the tough Marines?
I was there during the siege and witnessed the amount of firepower that
kept us from being overrun. It was a combination of unbelievable air
support, artillery, electronic listening devices to locate targets, and
an absolute refusal to yield the base. It angered me when it was
abandoned after being told for 4 months of hell that it was the anchor
of that part of the country. Why the Christ did we suffer that only to
give the NVA a talking point.

-----

William Reinhart
August 10
Thank you and your buddies for your courage and sacrifice. America never
lost a battle.

The problem in Vietnam was politicians willing to put Americans in
harm’s way fighting for a nation’s freedoms that did not care about
their freedoms and nor to put their lives on the line for them. I
understand that they were tired of 50 years of war being fought for the
political needs of countries far and away.and just wanted to be left
alone and let them figure it out.

America has still not learned this lesson.