Heat oven to 375°F. Combine ground flax seeds with 6 Tbsp water, let sit 20 minutes before using. Grease bottoms only of 12 regular-size muffin cups, or line with baking cups.
In a medium bowl, beat 2/3 cup sugar, the oil and flax eggs with a whisk. Stir in bananas and vanilla. Stir in remaining muffin ingredients just until moistened. Divide batter evenly among muffin cups.
Bake 18 to 22 minutes or until a toothpick inserted in the center comes out clean.
In a small bowl, mix 2 Tbsp sugar and 1/4 tsp cinnamon. Brush muffin tops with melted vegan butter, then sprinkle with cinnamon-sugar.