Reasonable permanence is acceptable. The same thing is often said about cadmium colors, too. This is because Cadmiums and Vermilion are very similar chemically. They are both metallic sulfides. The free sulfur or polluting sulfides may react with lead white.
Many people strongly warn against mixing cadmiums with lead white.
Vermilion was traditionally mixed with lead white for flesh tones. I m not the only one who thinks its day as an artist s pigment is long past. True, but there are plenty who say the same thing about Flake White, and the cadmiums. Of course, people need to know what they're doing. Mercuric sulfide is insoluble in water.