I’ve been living in Mexico for the past 7 years, and there are a couple of points I wanted to add about asking about the food:

– When you say you’re vegetarian, most people assume you just don’t eat red meat. At least half the time that I say I’m a vegetarian, the response is, “Oh, but this is chicken!”. That said, chicken broth goes in almost everything, ALWAYS in rice, and certainly no one sees that as “real” chicken. :-) Recently though, I’ve been able to get vegetable broth at the grocery store (but again, it doesn’t really occur to anyone to not use chicken broth).
– If people don’t really understand the meaning of vegetarian, they understand “vegan” even less…yes, the translation exists, but outside of counter-culture hippie communities, you’ll mostly get blank stares – it needs to be explained veeeery well, as most people are unfamiliar with the word and the concept…in my years in Mexico, I’ve come across a total of ONE vegan restaurant, and it was always empty.

Overall I’ve found being a vegetarian in Mexico quite easy (especially if you live in a city with several vegetarian restaurants), but being vegan in a little more difficult. Ades, the soy drink that you mentioned, is good but loaded with tons of sugar. I’ve been able to get organic soy milk only at Costco (at least organic soy milk that costs less than 40 pesos a liter), and soy protein at health food stores. The more exotic ingredients listed in many English-language vegan cookbooks are simply not available, so making anything a few steps away from a food in its “natural state” (like muffins, for example) is more challenging. Since there’s not really a vegan market down here, there are very few vegan “replacement” foods like soy cheese, yogurt, or ice cream, etc. Soooo…you’ve got to get creative! Fortunately there’s a lot of great food to get creative with!