thanks m_adam and zipit for your replies.
I am working in an ObjectData Plugin.
I am loading a mesh into my ObjectData (in Init) and need to add modifiers/subdivision in GetVirtualObjects.
After Zipits answer I noticed why it didn't work properly and that is that I added the tag to my ObjectData instead of the loaded mesh, the reason for that is that I would need the user to see the vertexmap but I can't expose the mesh.
I have now changed it so the vertexmap is created on my internal mesh, and that scales just fine when subdividing, so in that regard my problem is solved.
But I don't know if I am now able to expose the tag, and only the tag, somehow to the user.
My Plugin uses the vertexmap internally and I want the user to be able to use it for materials etc.
Is there any way?
Thanks for your help so far!