Bring a large pot of salted water to a boil and add the potatoes. Cook until they are tender when pierced with a fork, about 15-20 minutes.
Drain the potatoes in a strainer and allow them to cool slightly, enough to handle without burning.
While the potatoes are still warm, peel off the skins if desired and cut them into even slices or cubes, placing them in a large mixing bowl.
In a small bowl, whisk together the vinegar, olive oil, and a pinch of salt and pepper to create the tangy dressing.
Pour the dressing over the warm potatoes, gently tossing to coat all the pieces evenly, allowing the flavors to meld.
Add the finely sliced red onion to the bowl, mixing it into the potatoes for crunch and sharpness.
Sprinkle chopped parsley over the salad for freshness and color, then season with additional salt and pepper to taste.
Gently fold everything together to combine all the flavors without breaking the potatoes apart.
Let the salad sit for at least 10 minutes to allow the flavors to develop further before serving.
Give the salad a quick toss before plating and enjoy it at room temperature or slightly chilled.