Place potatoes in large pot with salted cold water covering the potatoes by 1 inch. Bring to boil over high heat, reduce heat to medium-low, and simmer until potatoes are tender, about 10 to 15 minutes.
Drain potatoes thoroughly, then add them back into the hot pot to dry completely. Drizzle the vinegar over the warm potatoes and toss gently until evenly coated.
Transfer the dressed potatoes to a large bowl. Cover the bowl and refrigerate until cold, about 30 minutes.
Mix the mustard with the sour cream, mayonnaise, celery salt and pepper.
Remove the potatoes from the refrigerator add the mayonnaise mixture and remaining ingredients. Gently mix with the cooled potatoes, taste and add more salt as needed. Cover, and refrigerate until well chilled, about 30 minutes.
Start cooking your potatoes in cold water, then bring them to a boil. This helps potatoes cook evenly.
Salt the cooking water well. The potatoes will absorb the water as they cook. If your water doesn’t have any flavor, neither will your potatoes.
Make sure not to overcook the potatoes or your salad can turn out mushy. When cooking, be sure to keep the water at a gentle simmer and not a rapid boil.
Use a fork (not a knife) to check for doneness. A fork should easily pierce the potato with a hint of resistance. A knife blade is sharp and will slip into the potato even if it is not fully cooked.
Let potatoes, drain well and allow the water to evaporate before drizzling with the vinegar mixture. This will help avoid soggy potatoes. You want them to be warm, but dry for this step. Let them dry completely in a single layer on a baking sheet. The water will escape, giving the dressing a better chance to coat to the warm potatoes.
Be sure not to skip dressing the potatoes with the vinegar and mustard while they are still warm, this will allow the potatoes to absorb all that flavor right through the potato.
Don’t add the mayonnaise or sour cream until the potatoes are chilled.
To avoid watery potatoes during cooling, place a kitchen towel under the lid of the bowl to absorb the condensation as they cool instead of falling into the potatoes.