This dressing straddles East and West, just like Vietnamese cooking. Its zippy, fresh flavor is great for a wide range of salads, from mixed lettuces to grated beets and carrots.
Zest limes into a small bowl using a Microplane grater. Squeeze juice from limes, adding vinegar as needed to equal 1/4 cup. Add lime juice-vinegar mixture, sugar, soy sauce, and oil to zest in bowl; stir until sugar dissolves. Season with salt and black pepper to create a balanced savory-tangy note. (I usually begin with 1/4 teaspoon salt and 1/8 teaspoon black pepper.) Stir in chile; let stand 5 to 10 minutes.
Refrigerate in an airtight container up to 5 days.