Preheat the oven to 350ºF and grease a 10-inch skillet with coconut oil. Drain the soaked millet and rinse well in a mesh strainer. Transfer the drained millet to a large food processor fitted with an "S" blade, then add in the chia seeds, water, zucchini, coconut sugar, salt, baking soda, and coconut oil. Process until the millet has broken down and a smooth batter is formed, about 2 to 3 minutes. Add in the vinegar and process again briefly to incorporate.
Pour the batter into the greased skillet and bake at 350ºF until the center feels firm when gently touched, about 30 to 35 minutes. Allow the bread to cool at least 20 minutes before slicing and serving. Store any leftover bread in an airtight container on the counter for up to 3 days, or in the fridge for up to a week.