Can you explain exactly how the US pulling out of the Vietnam war created a void, or a vacuum that took the country down?

Our neighbors are Laotian. They had a big Laotian bar b que last night and invited us. I don’t know if I had a good time because I couldn’t understand anything anyone was saying except, “You eat! You drink! You eat! You drink!” I ate. I drank. I not feel good today!

Seriously tho, they spoke English but not among themselves. I got into a brief conversation with a guy and I asked him what brought him to America. He said, “Well, I came in 1973…” I went “Uh oh…” when he was 12. He said that when the US pulled out of the war (he says, “You know..the Vietnam war?” Yes, I know.) it left a void that just collapsed in on itself. His family escaped and came here.
Can you explain that to me? What happened that would cause people to leave their native home where they’d lived for countless generations?