First, IT'S OKAY! Some of the deodorants I make use vanilla to make up the fragrance oil. (Tobacco Leaf & Amber, The Horseman, Clean Cut etc.) These deodorants are more likely to turn. There is nothing wrong with them except that they turn brown over time. The more they are exposed to air and light the more oxidation happens therefore the more they will turn.
Here's a nerdy explanation:The main discoloring culprit in vanilla is vanillin (4-hydroxy-3-methoxybenzaldehyde), an aromatic aldehyde that contributes a “strong, sweet, and milky scent” to our products. It’s found naturally in so many things, like vanilla beans, blueberries, coffee, red wine, smoked meats, strawberries, and more, but most of the vanillin we use is synthesized from ingredients like eugenol (found in cloves and cinnamon leaf), curcumin, wood pulp, or petrochemical sources as there’s just not enough vanilla in the world to satisfy our insatiable demand for the stuff.
So why does this discoloration happen? When exposed to air and light, vanillin oxidizes and starts to brown. (As an interesting aside, it can react with iron to turn purple!)