Here's a link to some info on that "unbelievably cheap" Mexican vanilla. As I think has been stated in PP, you can surely get good vanilla made from Mexican vanilla beans, however getting vanilla FROM Mexico itself should be something done with caution.
From the gourmetsleuth website: "Most of the vanilla extracts that come to the US from Mexico are synthetic. The best way to determine quality is price. If you are offered a large bottle for a cheap price it is most assuredly a synthetic product. Also, real vanilla is not "clear". No matter what the label may tell you if it is clear it is synthetic." http://www.gourmetsleuth.com/vanilla.htm
Another good site, including the history of vanilla in Mexico http://www.vanilla.com/html/facts-mexican.html
If you are buying Mexican vanilla please be safe! As has been discussed previously, in small amounts coumarin might not be toxic to YOU but it might cause an adverse reaction in someone you are baking for who might have a health condition.
BTW I just Googled LA VENCEDORA vanilla and according to the importer's website it IS FDA approved and courmarin-free http://www.leosimports.com/index.asp?PageAction=VIEWPROD&ProdID=398
I would be inclined to believe that if it's been shipped here legally and "above ground" i.e. subject to FDA regulations it's probably safe, but advance research is a good thing!
eta a list of Mexican vanilla manufacturers that have been flagged by the FDA to prohibit importation into the US due to coumarin : http://www.fda.gov/ora/fiars/ora_import_ia2807.html