There is virtually no regulation of ingredients or suppliers outside the US-Jamaica, the Dominican Republic, Mexico, etc. Vanilla really is pretty cheap, given the small amount generally used in recipes.

This may not be a 'comfortable subject', but the problem is real and perhaps the use of tainted vanilla may not even be traced back to a coumadin user's ingestion of a cake or cookies, but why take the chance? My mother-in-law is on coumadin and I couldn't bear the idea that my money saving could do her even the smallest amount of harm. It was hard to pass up a huge bottle for about $7, but I did. We were specifically advised not to buy the cheap vanilla due to this very problem. I was on a cruise to Mexico and while on the ship, there was a talk about shopping onshore. A simple fall on the sidewalk could lead to the same result. That type of bleeding could also then occur in the brain, heart, bowel, or stomach, equaling a medical emergency and possibly death. That may sound pretty harmless, but spontaneous bruising means that something is bleeding way too easily (perhaps even without a bump). Some patients are so sensitive that eating too much of the wrong foods-some salad ingredients, broccoli, etc.-can throw their level off enough that they bruise badly. 'Out of whack' coumadin levels can be very dangerous for these individuals. I can't speak to an exact amount that would cause a problem, but the offending ingredient, coumarin (basically a super cheap mimic rather than what should be used in the vanilla-making the product synthetic, not even real vanilla) is a relative of coumadin-a powerful blood thinner used by a sizeable number of older individuals. I use 2 tsp in a double batch of frosting.

how much would someone have to ingest for it to do harm.

Mexican vanilla extract near me