I'm reposting a slightly rewritten version of some information about Mexican vanilla that I previously posted on another thread on this subject.
Most of the "vanilla" sold in Mexico is artificial, and some is toxic. If it's cheap, it is NOT real.
You can't tell from the flavor. In fact, Cook's Illustrated magazine did a taste test with natural and artificial vanilla and the tasters were shocked to discover they couldn't tell any difference. In fact, in some cases they liked the fake stuff BETTER than the real stuff. (Unfortunately I can no longer link to the test report because they now require a membership to read articles on their site.)
I would never buy vanilla in Mexico... and I used to live 8 miles from the border, so I could easily do so. Frankly I won't buy any foodstuffs in Mexico, including processed foods, because a lot of them are unsafe. There is very poor oversight of food safety in Mexico due to corruption and lack of resources.
Here's another article about the potential for toxic ingredients in Mexican vanilla, by a well-known syndicated columnist who is known for doing his research:
http://www.straightdope.com/columns/011207.html
Mary
MouseSavers.com