Vertex alpha coordinates are welded in the same way with vertex colors. The only way to edit them is to use VertexPaint modifier.
If you enable Vertex channel display and select Vertex alpha in object properties, viewports will render aplha incorrectly. To fix it, export model to fbx and import it back.
This script imports vertex colors, but only their RGB data. Alpha channel seems to be missing. It is often used for material blending.