I can see that this topic has been posted a couple of times, yet none of the solutions seem to work for me. I'm trying to create a Houdini to C4D workflow that gives the highest quality results. I'll try to keep this very short and to the point. I'm sure once I have this alembic working inside Houdini it should work fine in C4d as well.
As you can see in the picture below, the cached surface is showing the velocity trails and renders perfectly.
Here is a render to show the motion blur working correctly.
Here is the alembic file imported in not showing any vector trails.
Here is how the render looks with incorrect motion blur (still in Houdini)
I've attached my file and would love some expert advice on this. Please let me know if you need anymore info.
There should be nothing wrong with the motion blur at all. The reason you cannot see the trailing vectors on your alembic loader is because it defaults Alembic Delayed Load Primitives, so it's not loading all the attributes, simply flick that to Unpack Alembic Delayed Load Primitives and you will see the trailing vectors.
I'd recommend not outputting one big alembic, and switch to per frame files. You geometry is changing point count every frame, so there is no data de-duplication going on to reduce file size. Per frame is lighter over a network or just off disk on general, and way less of an issue with crashing on cache write compared to one giant alembic.
Thank you so much! That worked perfectly Very good to know about the fact that alembic sequences more efficient. I'll be using them from now on.
For anyone reading this and wondering about how to get the velocity vectors working in C4D you simply need to drag in the XYZ velocity tags manually and will most likely need to adjust the scale depending on what units you are working in.
Son, this is amazing, thank you for sharing your discovery. If only trashtier c4d could figure out how to use the tools it already has at its disposal and not send artists on a 3 day binge trying to get it working.
Very good to know about the fact that alembic sequences more efficient. I'll be using them from now on. 
