Originally Posted by
Palanan
This is referenced in Age of Ultron, starting at 41:45, when they’re discussing Klaue:
Stark: “If this guy got out of Wakanda with some of their trade goods….”
Cap: “I thought your father said he got the last of it.”
Banner: “I don’t follow. What comes out of Wakanda?”
Stark: “The strongest metal on Earth.”
—And we end the scene with a closeup of Cap’s shield. The strong implication is that the vibranium which formed the shield came from Wakanda, and at the time it was assumed that was all there was.