Add the black beans, vegetable broth, soy sauce, olive oil, garlic, oregano, marjoram, rosemary, smoked paprika, onion powder, garlic powder, black pepper, and salt to a blender. Blend for about a minute, until the mixture is completely smooth with no whole beans left.

Pour the puree into a large bowl and add the vital wheat gluten. Stir with a spatula or your hands until a shaggy dough forms, then knead it in the bowl for 2 to 3 minutes, just until it comes together into a firm, slightly springy ball. Stop there, because more kneading makes the seitan tough.

Shape the dough into a log about 8 inches long and 3 inches thick. Wrap it tightly in foil, twisting the ends like a candy wrapper so no water can get inside.

Set a steamer basket in a large pot with about an inch of water below it and bring the water to a boil. Put the wrapped log in the basket, cover the pot, and steam over medium heat for 50 to 60 minutes, adding hot water if the pot runs dry. The loaf is done when it feels firm and springs back when pressed.

Take the log out and let it cool, still wrapped, for at least 15 minutes so it firms up. Unwrap it and slice it thinly across the log, about 1/8 to 1/4 inch thick, into strips.

While the loaf rests, make the tzatziki. Grate the cucumber, sprinkle it with a pinch of salt, and squeeze it dry in a clean towel. Stir it together with the vegan yogurt, grated garlic, lemon juice, dill, and salt, then chill it until serving.

Heat the olive oil in a large skillet over medium-high heat. Lay the seitan strips in a single layer, working in batches so the pan is not crowded, and sear for 2 to 3 minutes per side, until the edges are browned and crisp. Return all the strips to the pan, add the lemon juice and oregano, and toss for 30 seconds.

Warm the pitas, then fill each one with lettuce, tomato, red onion, and the seared strips. Spoon the tzatziki over the top, fold the pita around the filling, and serve right away.