For some reason, I’ve talked about this to a few people lately, even though I’m no longer a veggie and was never a vegan (my farts are bad enough as it is).
Anyway, most Thais are buddhist, but love eating meat and seasoning otherwise veg dishes with nam plaa (fish sauce), which is the local equivalent of salt. Their perhaps-spurious rationale is that they didn’t actually kill the animal, and not eating it would be a waste. I regard this as playing somewhat fast-and-loose with the conventional theories of supply and demand, but I am not an economist.
At some point, most Thai people will become extra-buddhist for buddhist lent, or joining the monkhood to make merit for a dead parent or some such (assuming they’re not in the small muslim or christian minorities). So every Thai person understands the idea of vegan food. This will always be respected; to give meat to a monk or someone who has requested “religious” food would be a grievous sin.
The magic words to unlock a world of vegan cuisine in Thailand are “Pom [if you're male]/ Dee-chan [female] kin ahaarn jair”. “Jair” is the key; it rhymes with “air” but has a slightly shorter vowel.
Any meaty-looking stuff in your resulting meal will be tofu (the smegma of Beelzebub, in my opinion) or mushroom. Soy sauce will be used for seasoning instead of fish sauce. If you want to avoid MSG, say “mai sai churot”. “Spicy” is “pet”, not spicy is “mai pet”.
There you are. Job’s a good ‘un. Of course, watch out for refreshing beers, most of which are clarified with isinglass like everywhere else in the world (and which would be strictly off-limits to monks, anyway).