Although most people don't believe there is a difference between plain and vanilla varieties of a product (e.g. yoghurt), there is in fact a difference.
Plain means without any flavor whatsoever. And vanilla is a flavor, even though it's a very mild one. Vanilla flavoring is derived from orchids of the genus Vanilla native to Mexico. Author Frederic Rosengarten, Jr. described the vanilla flavor in The Book of Spices as "pure, spicy, and delicate" and its complex floral aroma depicted as a "peculiar bouquet." Despite its high cost, vanilla is widely used in both commercial and domestic baking, perfume manufacture and aromatherapy.