Combine vinegar, sugar, and salt in a small saucepan. Heat on low, stirring occasionally until salt and sugar dissolve.
Add water. Let mixture cool.
Add red onions to a jar or bowl (with lid) along with optional flavorings.
Pour liquid into jar. Let sit at room temperature for one hour, then refrigerate. (As the onions pickle, they will release some of their water, and you will ultimately end up with a little bit more liquid in your container.)