Put the potatoes in a pot, cover with cold water and add some salt. Cover, bring to a boil, lower to a simmer and cook until easily pierced with a fork, 12 to 15 minutes. Drain the potatoes and place them in a bowl. Sprinkle the potatoes with the vinegar and let cool, then halve them lengthwise.
Mix the creme fraiche, mayonnaise, 1 teaspoon salt and 1/2 teaspoon pepper in a small bowl. Add the creme fraiche mixture to the potatoes and, with a rubber spatula, very gently fold the mixture into the potatoes, taking care not to break them up. Add the chives and radishes and toss again. Just before serving, sprinkle with the dill.
The potato salad can be made a day in advance; just bring to room temperature before serving.