Prepare your chosen vegan protein (tofu, seitan, or mushrooms).
Make the marinade by combining soy sauce, garlic, ginger, sesame oil, brown sugar, and vinegar.
Marinate the vegan protein for at least 30 minutes (longer for richer flavor).
Cook the vegan bulgogi using pan-frying, grilling, or baking (5-15 minutes depending on the method and thickness of the protein).
Assemble the sandwich with your choice of bread and toppings (kimchi, pickled onions, shredded carrots, vegan mayo, sriracha).