Reddit mentions of Companion Mun Chai Ya Imitation Mock Roast Duck, Vegan

Vegan: No parts made from animals (fish included). This includes dairy. For some real strict vegans, honey might be included.

Gluten free: Anything made with wheat, barley, or rye. This does include soy sauce and by extension a whole bunch of other Asian sauces that use soy sauce, such as oyster sauce, XO sauce, etc. Also, beverages made with wheat, barley, or rye, like beer.

In general, try to have a balance between a starch dish, a protein dish, and a salad of some sort. If you stick to rice, your protein dish is the only one that will have to be what you called "Normal, Vegetarian/Vegan and Gluten-free". You can double up on some days to make it easier for you. For example, your Normal dish can also be Gluten-Free. It'll free up some time if you just make two large batches of food.

Ask your friend how many people are gluten free, how many are vegan, how many are vegetarian. It'll help you balance what you need to make for the day. If you only have 2 or 3 vegans and you made 10 portions, that might be a lot of wasted food (some non vegans might eat it as well).

I've worked with a few gluten free soy sauce alternatives. Liquid Aminos is pretty good. Kikkoman is now selling a gluten free traditional soy sauce, but that might be expensive for 20-30 people.

For your vegan crowd, I haven't found a really good replacement for fish sauce yet. The closest is mushroom ketchup, which was historically made by the English who were trying to recreate fish sauce back in the day. Mushroom ketchup is also not gluten free, if you have someone who is both gluten-free and vegan.

If you had a way to keep soup hot for a long time (like a large crock pot), I would recommend making a congee bar. To keep the base safe for everyone, use vegetable stock and mushrooms to replace the usual chicken meat to flavor the rice. Have it surrounded by the usual ingredients of fried onions, fried garlic, a bottle of soy sauce, some shredded pork, scallions, etc. The unused toppings can be saved for noodles the next day.

Also, your vegan/vegetarian crowd might appreciate "Mock Duck" A local thai place has it as one of it's options. I'm a meat eater and I really enjoy it, as does my vegetarian friends.

For the actual cooking, I like to make a list on what needs to be done and when. Cooking 4-6 dishes can be difficult. If one slips your mind, you can end up behind schedule trying to fix a mistake. Try to see what can be made in advance and stored for refrigeration. Buy a lot of sternos and trays to keep your food warm.

Good luck!