A button that binds vertices exclusively to selected bone/envelope (reverse exclude but with less vertex selecting). The name may be too close to the first button, so maybe something like "Only include" could work.
Middle button with a circle around (or circular brackets).

Other users have mentioned how slow this "masking" becomes as the mesh gets more complex. I use this feature a lot in combination with envelopes but it feels very underdeveloped.
Other modes of masking could be "Contained Element" where only vertices of a single submesh Element/hull is included/excluded. This would be great for hard surface skinning.