As above, and blender.stackexchange.com/questions/118506/can-i-use-vertex-alpha-to-blend-2-textures. Possibly related / fortuitous developer.blender.org/T58711.
In my use case vertex RGB channels will hold color and Alpha will blend other stuff. Is anything like the pictured / annotated planned to better modify and access the available Alpha data?
Vertex Color Master improves the Alpha paint fidelity by transferring values from one RGB channel. Workflow is paint ‘Alpha’ first. Transfer to actual Alpha channel. Paint RGB. Could be improved though as above. Access via nodes is still unsolved AFAIK.