The idea is good but problematic. I have not tried but think of several problems:
1. greasy materials, like vaseline, has a tendency to spread. it will not give a sharp boundary.
2. the paint is also grease-like. If the paint is applied in the form of a spray, it will try to dissolve part of the grease. In fact, a small amount of grease will not prevent paint to stick (paint will stick in spite of the grease).
3. The paint will not be flammable after it dries. The solvent is highly volatile and flammable.
4. Most paint will not corrode copper. In fact, the green solder mask widely used for solder resist is a paint.
5. Common paint will not stand the heat of a soldering process. The paint will burn (some may be more resistant).
6. How do you plan to pull off the paint from the vaseline coated area? It may peel off like a skin.