Heat the potatoes to boiling in enough lightly salted water to cover. Reduce heat to low, cover, and simmer 10 minutes or until the potatoes feel tender when poked with a skewer. Drain.
Meanwhile, in a large bowl, whisk together the vermouth, scallions, vinegar, lemon juice, and olive oil. Stir in the horseradish and parsley. Fold in the hot potatoes and trout. Serve warm or at room temperature. Can be made up to 2 days ahead.