Steam the potatoes in a steamer basket for 20 minutes, or until fork tender.
Meanwhile, whisk together the vinegar, mustard, and olive oil in a large mixing bowl.
About 2 minutes before the potatoes are done, toss the peas into the steamer basket. When the potatoes are cooked and the peas are heated through, add them to the bowl with the dressing. Add the herbs and toss everything together. Season generously with salt and pepper to taste. The potatoes can take a lot of salt! Toss again and refrigerate until just chilled.