Cover potatoes with salted cold water by 1-inch in a 4-quart pot and simmer until just tender, about 10 minutes. (They will continue to cook after draining; do not overcook or they will break apart.)
Drain potatoes and rinse under cold water. While potatoes are still warm, gently toss with vinegar and salt. Cool potatoes to room temperature, stirring occasionally, and gently toss with oil.