I'm not getting a gradient between edge loops on the mask, since the change to Color Attributes are there settings I need to change when creating the mask?
I made the Domain = Vertex, the Data Type = Color and the Color solid black (HSV=0).
I'm thinking I need to change the Domain to Face Corner, although just thinking about it, when switching to Vertex Paint the meniscus turned black so maybe the Color should be RGB=0.8?
Any help would be appreciated :)
Your Attribute settings should work; there should automatically be a gradient between black Vertices and white Vertices.
If you'd use the Face Corner Domain, and use Face Selection Masking, you could create a sharp separation from black to white.
Can you share a screenshot, please?