In a small microwave-safe bowl, whisk water, rice vinegar, and salt until well-combined. Place bowl in microwave and heat water at 100% power in 1 minute intervals until 209 to 212ºF (98 to 100ºC), about 4 minutes total.
Using oven mitts or a kitchen towel, carefully remove bowl from microwave and gently slip the egg into water. Return bowl to microwave and cook at 80% power until the whites are fully set but yolks are still soft, about 1 minute and 20 seconds.
While eggs are cooking, place a paper towel on a plate; set aside.
Using a slotted spoon, carefully lift egg from bowl, allowing any excess liquid to drip back into the bowl. Transfer to paper-towel lined plate and, using a paring knife or scissors, trim whites if desired. Serve immediately.