Soak rice in cool water for 2 hours. Swirl around to release some of the starch and drain.
Bring 1 ¾ cup/412ml water to a boil over medium-low heat in a medium pot. Add the soaked rice and the vinegar. Stir once to distribute.
Cover and cook for 10 minutes. Remove from heat. If the water has not been full absorbed, leave covered and allow the residual heat to steam the rice.
Uncover and fluff with a fork. Set aside.
In a medium saucepan, combine the coconut milk, sugar, and salt. Stir to dissolve the sugar
Cook over medium heat, stirring occasionally until simmering.
Remove 1 cup/235ml of coconut milk mixture.
Pour over rice and stir to combine. Allow to sit for 15 minutes for the milk to absorb.
Return the remaining coconut milk mixture to heat. Cook at a simmer/low boil, stirring frequently, until volume is reduced by half, about 15 minutes. Regular stirring will prevent burning. For an alternative to this step, use coconut sweetened condensed milk – see notes.
Once dipping sauce has reduced and is thick, remove from heat.
Peel the mangoes and cut the sides off the pit. Slice each side into 1/4”/6mm or thinner slices.
Have all components within easy reach and a large shallow bowl or pan of cool water.
Dip one rice paper sheet in the bowl of water for 7-10 seconds, making sure to submerge it completely. There is a rough side of the paper and a smooth side – keep the rough side up so the roll will hold together.
Place the wet rice paper on a plate or cutting board with the rough side up. The rice paper will not be fully flexible after the initial dip in the water, but as you assemble the filling it will soften for proper rolling with becoming unruly.
Sprinkle a 3”/8 cm line of toasted sesame seeds in the center of the paper.
Place 3 or 4 slices of mango on top of the seeds.
Place two large scoops of rice on the mango.
Fold the sides of the rice paper over the ends of the filling.
Take one edge and wrap it fully around the filling. Roll the filling towards the remaining end.
Repeat with the remaining ingredients.
Serve immediately with the sauce for dipping or drizzling over the rolls.
Sauce can be stored in the fridge for up to 1 week. Rice with coconut milk can be made up to 1 day in advance. Reheat covered in the microwave on medium heat for 2 minutes. The texture will be slightly softer and have less of the classic chew if reheated.