Mash the ripe bananas in a small bowl then transfer to a saucepan. Add all other ingredients (apart from the vanilla).
Place the saucepan over a medium heat and bring the mixture to a simmer while stirring. Simmer for about 5 minutes.
Remove from the heat and let cool.
Strain the syrup through a fine mesh sieve over a bowl. Add the vanilla extract into the bowl and stir, then pour the contents of the bowl into a mason jar or similar storage to keep the syrup.
Store in the refrigerator for up to two weeks.