Cover potatoes with salted cold water by 1 inch, then simmer until just tender, 10 to 15 minutes. (Potatoes will continue to cook after draining; do not overcook or they will break apart.)  Drain potatoes and rinse under cold water until slightly cooled. Halve lengthwise and while still warm gently toss with 1 tablespoon vinegar. Cool potatoes to room temperature, then season with salt and pepper.  While potatoes cook, coarsely chop jalapeños and pulse in a food processor with cilantro, shallots, garlic, oil, and remaining 3 tablespoons vinegar until finely chopped.  Toss potatoes with salsa.  Notes: Cool the potatoes before tossing them with the salsa so the herbs won't discolor. Salsa may be made 1 day ahead and chilled, covered.