Here's some mentions of vibranium and Wakanda I could find in the films:
Captain America: The First Avenger
Howard Stark: Vibranium. It's stronger than steel and a third of the weight. It's completely vibration absorbent.
Steve Rogers: How come it's not a standard issue?
Howard Stark: That's the rarest metal on earth. What you're holding there? That's all we've got.
Avengers: Age of Ultron
Tony Stark: If this guy got out of Wakanda with some of their trade goods...
Steve Rogers: I thought your father said he got the last of it?
Bruce Banner: I don't follow. What comes out of Wakanda? [looking at Steve's shield]
Tony Stark: The strongest metal on earth.
Ultron: ...Upon this rock I will build my church. Vibranium.
Ulysses Klaue: You know, it came at great personal cost. It's worth billions.
Captain America: Civil War
King T'Chaka: When stolen Wakandan vibranium was used to make a terrible weapon, we in Wakanda were forced to question our legacy...
Emphasis mine. From these quotes, two things seem clear:
Wakanda is known for having, and sometimes trading, vibranium.
Vibranium is believed to be extremely rare.
Based on these facts, it seems likely to me that the US government (or Howard Stark) simply bought the vibranium from Wakanda. Likely it cost a fortune, and the buyers were led to believe that they got absolutely all of it.
This was perhaps a diversionary tactic by the Wakandans to prevent invasion; once the secret got out that they had vibranium, foreign nations would take interest. Rather than destroying these foreign nations (and revealing their power to the rest of the world), Wakanda must have decided to take a more peaceful approach by pretending to sell off all their vibranium to the Americans.