Cook potatoes with skin in boiling water for about 20 minutes. Rinse beans and corn in a sieve and drain well.
For the vinaigrette, mix vinegar, salt, pepper and 1 tablespoon of sugar. Fold in the oil. Wash parsley, shake dry, remove leaves and chop finely.
Drain the potatoes, quench and let them cool down a little. Peel and slice the potatoes and add them to the vinaigrette. Fold in beans, corn and parsley. Season to taste with cayenne pepper.