The attribute node is not supprted as of 2.0.112. As a workaround/alternative it was suggested on the Discord that I should be able to use RPR Lookup set to Vertex Color to get similar effect for use in shaders.
This doesn't seem to work.
I have attached screenshots from and example where I have exported an object from Houdini using alembic with vertex colors. In Blender I have renamed the attribute from "Cd" to "Col" to be Blender standard. EEVEE and Cycles can use this using the Attribute node.
Vertex colors in RPR:
Preferred solution would be using the Attribute node as the other renderers. Using RPR lookup would be an ok workaround for this particular application.
I have made a similar example with vertex colors from H via .abc . This also works in cycles and eevee but renders black in RPR. In this case I have tried to make different names for the attibute. Both "Cd" or any other name works for the other renderers if the same name is given in the Attribute node.