Alpha only Blend Mode for Vertex Painting

This is just doesn't make sense, having two blend mode for modifying vertex alpha channel.

Can we get one Blend Mode just for painting vertex alpha channel value, instead of two separated blend mode? Maybe Called it "Affect Alpha Only" blend mode

Vertex Alpha are grayscale, just changing the Color value to darker value or brighter value should be enough. The current vertex alpha painting workflow are convoluted. With a bonus of a new Vertex Painting brush called "Alpha Only" would be awesome