To a small saucepan add red wine vinegar, water, salt, and sugar. Bring to a simmer over medium heat and stir to fully dissolve salt and sugar. Taste and adjust flavor as needed, adding more salt or sugar to taste.
Pour the brine over the onions, ensuring they are fully submerged. If needed, add more vinegar or a little water to cover.
Seal well and shake to combine. Then refrigerate for at least 1 hour (or at least 30 minutes if in a hurry). The flavors will deepen and intensify the longer it marinates. Best flavor is achieved after 24 hours.
Will keep in the refrigerator for 2-3 weeks (sometimes longer). Not freezer friendly.