Im struggling with getting my motion blur to read in maya, on an alembic exported from Houdini with a changing point count. Ive seen a lot of information about promoting velocity to a vertex attribute, or transferring vel to Color, then converting that to a vertex attribute and renaming it v. Nothing is getting velocity working in vray. If anyone has any additional insight, Id appreciate it!

assuming you are using Vray Proxy to load .abc file (as Maya itself is very bad at passing attributes to renderers or even has issue loading some data from alembic like particles or curves so it's usually better to bypass loading data to Maya altogether)

but even with Vray Proxy in my experience Vray has had issue with rendering .abc file with changing topology in the past so it's likely it still has
however you can export .abc sequence from Houdini and load in Vray Proxy node using frame token and it should render fine with velocity motion blur (just with usual v attribute on points)
I forgot what is the frame token syntax in Vray for Maya, but I assume something like: <frame04>

Check out these for some possible solutions:
https://forums.odforce.net/topic/25447-exporting-alembic-from-houdini-to-maya-no-motion-blur/ [forums.odforce.net]
https://www.toadstorm.com/blog/?p=346#more-346 [www.toadstorm.com]