Place a steamer basket in a saucepan filled with 1 inch water. Bring to a gentle boil. Add potatoes. Cover, and cook just until tender, 15 to 20 minutes, tossing occasionally.
In a serving bowl, combine vinegar and Dijon; season with salt and pepper. Add hot cooked potatoes; toss. Let cool, tossing occasionally.
Add oil and parsley to cooled potato mixture. Season with salt and pepper, and toss.
When steaming a lot of veggies, do them in batches, making sure the water level doesn't get too low. Watch out for the hot steam when you remove the basket.