How to find direction Vector for a collision surface?

Basically I have two Vector2s: velocityBefore and velocityAfter, and I want to find a vector of a collision surface like shown in an image. It needs to point in a direction of velocityAfter, so if the ball on the picture would come in a reverse direction, the surface vector would point the other way. Is there a build-in method I can use for this or maybe it can be achieved just by some formula?