You can construct a mask for vertex colors in the same way you’d construct masks for anything else. Looks to me like what you are trying to accomplish cound be done by constructing an object space gradient, and then using a step node to turn an axis of this gradient into a black and white threshold.
Here’s a video that covers how to make an object space gradient you can use to construct such a mask.
Once you have the mask, you simply multiply your vertex color by it, or use it as an alpha for a lerp, or however you want to operate your mask.