Hi voice-item,
Your question is different. If I understand correctly, you would just like the Vertex map data as a mask.
Here is an example. Render the file. Then go to the Render Settings and use the Overwrite Material. Render again.
I have set up a material that only shows the Vertex map. The material will show any Vertex map with the same name in the material entry.
If you have follow-up questions that are different, please open a new Forum Thread; otherwise, things become hard to search and read. Thank you.
All the best