Formula: Vermilion is an alchemy formula in The Witcher 3: Wild Hunt that is needed to craft vermilion.
It's a possible spawn at a random diagram location or purchased from the following merchants:
- Gremist at Gedyneith
- Merchant in the Perfumery in Beauclair
- Wine Merchant at Chuchote Cave
- Wine Merchant at Dun Tynne Hillside