In a small saucepan, whisk together vinegar, sugar, and salt. Bring just to a simmer over medium heat, whisking constantly until sugar is dissolved. Remove from heat, cool to room temperature.
Peel and seed the mango. Chop mango flesh into chunks, place into a blender with cooled vinegar-sugar mixture and vegetable oil. Blitz until very smooth, chill until you’re ready to use it.
To serve, spoon sauce into a pastry bag or a sauce bottle (pictured). Cut the tip off the pastry bag (if applicable), squeeze sauce over prepared sushi, as desired.