It's entirely possible to scratch strong metals, even with weaker materials. One example is using water to cut metal. It's about concentration of force, not strength. See this question on the physics Stack Exchange: Is it possible to cut harder material with a less hard material?
The pressure that a water jet can produce during cavitation; the momentum of.a soft sand grain burrowing into a hard coating; the heat generated when laser light hits a surface; the heat of electrons decelerating as they hit an anode - all these and more can be mechanisms of material removal which can be used to shape an object - "cut" it. Typically the mechanism is that local pressure, while exceeding the yield stress of the "soft" material, cannot be dissipated because of the speed of impact.
This exact question was also already asked on the movies Stack Exchange: Vibranium material properties.
We can clearly see the claws are sharpened to blades, the shield is a perfectly flat surface. The claws would rightfully so scratch the shield as they are the same density but the blades are sharp points.
If it helps think of it in terms of real world glass. You can sharpen glass to a point where it can cut through other glass not too dissimilar to a diamond cutting through glass.
Here's another related question: How does Vibranium work?
Whether normal Vibranium can be converted into Anti-metal is unknown, but the Wakandans do have small supplies of this Savage Land Vibranium on hand and have equipped the Black Panther with it.
The material coats his claws and often other weapons he uses, allowing him to tear through metallic substances with little effort.
There is also a ton of info in these questions: