My significant other asked me to reply to your email-I am the vegan guy but am not alcmaeon. I have to say I was surprised at the number of places that I could get vegan food at. There was even soy milk for sale! I did eat a couple of dicey meals where the wait staff said the food was vegan but I suspected it not to be by its flavor or consistency. However if you are ok with dairy and eggs you will have no problems at almost any place on Ambergris. By far the best meal I had was at Sweet Basil were the staff was very open about what was in the food and even knew the "correct" questions to ask me (did I eat butter, mayo, etc). Also I did eat at Mangos where the tofu dishes were simple but tasty- they sliced silken tofu and added various sauces to the dishes--I liked the "Tropical Tofu".
Also I was able to go on a couple of the all day charter trips :Lamanai with Seaducded, a Manatee trip with SeaReious (Fantastic trip!) and the Blue Hole with Amigo Del Mar. On all three I got tons of Belezian Beans and Rice and was happy with that. Also the grocery stores had many US brand foods so I could get
cereal, peanut butter (Jiff Crunchy) and I ate a ton of fruits and vegies.
Overall San Pedro gets a good grade, the locals seemed knowledgeable about vegetarians (the Searious people even asked if any of us were vegetarians before I could bring it up). Also the locals didn't seem to think I was totally insane for not eating meat, dairy, eggs, etc which is an attitude I have found in Mexico--they look at me like I am just the most Loco Gringo ever.
Enjoy your trip!