Is it so expensive that it’s worth the risk?
Vanilla essence is normally extracted and sold in ethanol. Ethanol’s a good preservative but over time it evaporates from all but the most securely sealed containers, so I wouldn’t trust an opened container of that age.
The issue with any food of that age isn’t whether the food itself has undergone some chemical reaction, it’s whether it is growing microbes. Even the most hostile environments, such as pure sugar crystals, are capable of growing microbes at an extremely slow rate. Given 15 years I would assume that any opened food source will be alive even if it’s not detectable.
The problem is that there’s no reliable way of detecting these things outside a micro lab. The don’t always produce odours or other signs of spoilage. What they do often do is produce some really hideous toxins that accumulate over time.
IMHO it ain’t worth it unless the stuff is worth at least $100 a bottle. The risks would be much less if the bottle has never been opened, but quite frankly any food source that old is suspect.