Masking volumes

I Need to mask volume on a object. in my case i want to bring the density to zero, so its fully transparent, but i still want to keep the density at the other areas of the object.
image

If you want a volume fully transparent on a part of the object you can use a boolean to remove a part of the geometry or if you want a volume that progressively become transparent you can use a gradient texture for densitiy of the principled volume

tried with vertex paint, aint that supposed to work? considering a volume is 3D it does not make much sense.