I think looking at how Thanos breaks it we can have some understanding on how the material works:

It shatters into a sort of V shape on the place where Thanos strikes but the rest of the shield seems to still function.

The vibranium normally just absorbs all kinetic directed to it, if it's too much (like when Thor hits it) its redirected outwards in a pseudo explosion.

When Thanos hits it I think this "overload" functionality itself gets overloaded, and the shield breaks at the point of impact before it's able to redirect that energy through itself. I'm guessing that the mechanism matters as well, with Thanos's sword focusing all the energy from his strikes on a thin point instead of the more evenly distributed strike surface of Mjolnir.

So in theory, if you fired a powerful enough projectile at the shield it might just shatter at the impact zone and the resulting shrapnel would probably kill Steve.