Vanilla is also a note that barrel ageing stamps on wine, and it seem folks love their vanilla! From Ice Cream to soda pop, vanilla is one of our favorite flavors, so why not in wine?
Oak is rich in many aromatic compounds and vanillin, the aroma of vanilla, is one of those compounds. When a wine spends time in a barrel, many of these compounds leech out into the wine, adding layers of aromas and flavors.
After several years, the layer of wood that interacts with the wine loses almost all of these compounds -- at which point the barrel is referred to as being a neutral barrel, and is generally traded in for a new one.