Your scene is instantly crashing the latest master (3.0) but I can import the abc and something is weird / wrong. If I add a Geometry Node modifier to inspect the attributes on the mesh it looks like this
As you can see there are 4 different vertex color layers, each with RGBA channels.
Usually velocity vectors are living on points, not on face corners and they’re a vector3 (XYZ or RGB… not vector4 like RGBA). And then all 3 components of the velocity vectors should be in a single vector3, not in 3 or 4.
Something went wrong. Blender can actually render motion blur coming from a velocity attribute fine, even when there’s no actual movement on the points. Just try this scene here to see what I mean.
motion_vectors.zip (116.1 KB)
This icosphere is blurred although there is no animation or movement on it. Just a velocity attribute.
This is what motion vectors should look like
P.S.: I just noticed that in Blender “points” are called “vertices” and “vertices” are called “face corners”. This is strange because in my world vertices are points that are part of a poly (i.e. face corners), whereas points are just erm… points. A single vertex that is not part of anything “higher” like a poly is a point IMHO. I wonder how the naming will be as soon as particles make their way into GN