Ok, so I found what I think is the simplest workaround for my use case for VDMX sources. Basically white objects/elements on a transparent background. I've used the node Mask Image by Brightness (vuo.image.mask.brightness) and treated the black mask layers as if they were chroma key green/blue. But it would much simpler and cleaner to have access to an Apply Layer Mask node.
It puzzles me why such basic nodes are missing from Vuo. It seems obvious that when Apply Mask (vuo.image.apply.mask) was created that Apply Layer Mask (vuo.layer.apply.mask) would be created at the same time? I understand there are complexities is working with Image and Layers but as I've said before having consistency in how functionality is rolled out is important particularly for new users.