As long as the bullets don't hit dead on, they're going to retain some component of velocity tangent to the shield. Depending on how you decide to apply the magic 'vibranium' properties that deflection is either allowed or not allowed. There's not really a concrete description of how the shield works that's infallible canon as far as I know (but I'm not a reader of many comics) so there isn't really a way to say whether bullet deflection really makes sense or not.

I can tell you, however, that a real shield like that would either pulverize bullets that can't penetrate it if they strike close enough to straight on or be shattered and/or deflected whole if they were grazing blows.

Real world bullet-energy absorption is what you see in bullet-resistant plastics like Lexan. They don't shatter like glass, but they permanently deform ("yield") and disperse large amounts of energy The deformation is pretty localized so they can take a lot of damage before suffering a real failure. But if his shield were made of Lexan for instance, he'd need at least an inch thick or so to be really safe. The nice thing is that he could certainly paint it with metallic paint or use a dye during the manufacturing process. But it certainly wouldn't bounce well. It would work more like a conventional shield -- useful when held between you and your enemy and little else. Good thing Cap is willing to carry firearms.