Mix the yeast, sugar, and 1/4 cup warm water. Set aside until foamy and doubled in volume.
Mix the flour and salt in a stand mixer. Add the proofed yeast and melted vegan butter, then mix on low while slowly adding 1 1/3 cups warm water.
Increase to high speed and knead for 5 to 6 minutes. Divide the sticky dough into four balls, place on an oiled baking sheet with space between them, cover, and rise for 45 minutes or until doubled.
Bring 6 cups water to a boil in a large pot and carefully add the baking soda. Preheat the oven to 375°F.
Score each dough ball and boil them one at a time for 1 to 2 minutes. Transfer to parchment-lined loaf pans, placing two balls side by side in each pan.
Sprinkle with coarse salt if desired and bake for 15 to 20 minutes, until deeply golden brown.
Notes
Boil the dough balls one at a time so the alkaline bath stays hot.