Line slow cooker with parchment paper and turn on to high heat to warm up while you prepare batter.
Combine non dairy milk and apple cider vinegar. Set aside.
In a large bowl combine flour, baking powder, salt and cinnamon, whisking to thoroughly combine.
Add mashed bananas, yogurt, sugar and vanilla to milk mixture and stir to combine.
Add wet ingredients to dry ingredients and stir just to combine.
Add walnuts and fold into batter, if using.
Carefully transfer batter to warmed slow cooker and spread in an even layer using a spatula. Cover and cook for 1 hr 45 mins to 2 hours. Time will vary based on your particular slow cooker. Use a skewer to check for doneness by inserting in middle of loaf. It will come out clean or with only a crumb or two when the loaf is ready. The loaf will also be lightly browned.
When loaf is ready, turn off slow cooker and allow to cool a few minutes before carefully lifting it out using the parchment paper. Transfer the loaf to a wire rack for cooling. Cool completely before slicing.
Store in air tight container for 2-3 days.