In a saucepan over medium-high heat, heat 2 cups water, then stir in sugar, fish sauce, and vinegar. Stir to dissolve sugar.
If not thickening the sauce, let it cool, then stir in ginger, garlic, and lime juice. Add Sriracha and/or chile.
If thickening the sauce, after sugar dissolves in Step 1, ladle 1/4 cup of mixture into a bowl and stir in cornstarch. Pour back into saucepan and simmer, stirring, until as thick as maple syrup, 1 to 2 minutes. Let cool and add remaining ingredients as in Step