I think comic book bad guys have the right idea, aiming their weapons directly at Captain America's shield. That's probably his weak point.

If Captain America's shield is vibranium, then... Hawkeye's shield is Quicksilver

Captain America's shield was made of Adamantium. What was Hawkeye's shield made of? Quicksilver.

What's Captain America's shield made out of? What's Captain America's shield made of? Vibranium. What's Hawkeye's shield made of? Quicksilver.