Combine white sugar, salt, water, and sweetened condensed milk to a small saucepan on the stove. Do not add the vanilla extract yet.
Whisk together and heat on low-medium heat for about 5 minutes. Whisk continuously and do not leave the stove. Heat until the mixture foams and all the ingredients are dissolved. You don't want a rolling boil, just a gentle foaming.
Once the ingredients are dissolved together, remove saucepan from heat. Allow the syrup to mostly cool in the pan. Add the vanilla extract and stir.
Once mostly cool, pour the syrup into a glass jar or bottle for storage. Allow to completely finish cooling on the counter top before storing in the fridge.
When ready to use, shake the bottle or jar of syrup, pour into your beverage, and enjoy!