Set a steamer basket in a large pot. Fill with enough salted water to come just below basket. Bring to a boil; place potatoes in basket, and reduce heat to medium. Cover, and steam, gently tossing occasionally, until crisp-tender, 10 to 15 minutes.
Meanwhile, combine vinegar and scallions in a large bowl; season with salt and pepper. When potatoes are cooked, transfer to bowl with vinegar mixture. Toss to combine; let cool, tossing occasionally.
When potato mixture is cool, mix in oil; season potato salad with salt and pepper.
To store, refrigerate, up to 1 day.