Preheat the oven to 350ºF and line a loaf pan with parchment paper. In a large bowl, use a fork to mash the bananas well-- you should have a little over 1 cup of mashed banana. (Note: if using bananas that were previously frozen, do not drain the liquid from them when they thaw-- that moisture is important to the overall texture of this bread.)
To the mashed bananas, add in the almond meal, arrowroot, coconut sugar, baking soda, salt, vanilla, and cinnamon and stir until a relatively smooth batter is formed.
Stir in the vinegar last, so that it can react with the baking soda, then immediately pour the batter into the lined pan. Smooth the top with a spatula then bake at 350ºF until the top of the loaf has begun to rise and crack, feeling firm to the touch, about 45 minutes. All the loaf to cool completely before cutting.
As with most eggless bread recipes, it's not unusual for the center of the loaf to fall or sink a bit, but the entire loaf should be cooked throughout and have a bread-like texture. (Definitely a step up from my previous vegan & paleo banana bread!) Store the leftover bread in an airtight container on the counter for up to 3 days, or in the fridge for up to 10 days.