Yes, but not many.
It's possible that Bruce, Tony Stark, and possibly Howard Stark already know about the Wakandan vibranium. Besides them, there are more people who tried to intrude and steal vibranium from Wakanda. One of them is Ulysses Klaue, a guy Ultron gets Vibranium from.
Here are some references.
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.
Now coming to your second question,
Then how can T'Chaka make such a statement at the time of Civil War?
Well, there were too many casualties in Avengers: Age of Ultron, which were caused by the weapons made by Ultron using Vibranium. It may not be known to the world, but it was to some people.