There's more than one chemical compound in real vanilla extract, but the major one is vanillin, and it's responsible for most of the flavor. There's a variety of ways to make synthetic vanillin, but the most efficient (and cool, in a very nerdy way) method used in the U.S. involves some wild chemistry and wood pulp waste. A less elegant approach uses coal tar as the starting chemical, but the final product vanillin is completely identical to vanillin from vanilla beans in both cases.

So, the vanillin part of synthetic vanilla is EXACTLY the same as real vanilla extract, but the more subtle other flavors have to be added to the fake vanilla. One way to do this is by making an extract from Tonka Beans which unfortunately contains coumarin, which has nasty blood thinning abilities (it’s a really close chemical cousin to both Warfarin and rat poisons). Due to both the blood thinning properties, and causing liver and kidney issues, the FDA banned coumarin as a food additive in the 1950's. Unfortunately, most of the third world doesn't have effective food additive regulation, and Tonka Beans are one of the best ways to juice up synthetic vanillin to make fake vanilla extract.

Apparently, the percentage of coumarin found in vanilla extracts imported from Mexico and the Caribbean has significantly dropped as of late, which is good. There's still plenty of other nasty things floating around in fake vanilla, like red dyes that are banned in the U.S.

Anyway, the important part is all health concerns aside, virtually all "vanilla extract" that people bring home from Mexico is fake. During my food and drug law class, we had a guest from Michigan Department of Agriculture who said that he had yet to test a single bottle of vanilla extract bought in Mexico that was real, and it was completely safe to bet that every large bottle (i.e. the 16oz or so plastic ones) for sale to tourists, in all of Latin America contained fake vanillin with random additives.

My take on the matter is if you're going to buy fake vanillin anyway, you might as well buy it in the U.S. so you don't have to sweat the coumadin/red dye/additive issues.