Put quail eggs in small saucepan. Add 1/4 cup vinegar, then add enough water to just cover the eggs then bring the water to a boil.
Boil eggs for 5 minutes, then pour off the water. Add ice water to cool the eggs.
When eggs are cooled, peel the eggs. Cut in half and remove the yolks.
Mash the yolks in a small bowl until they are smooth. Add mayonnaise, mustard, remaining 1/2 teaspoon vinegar, and sugar. Mix until smooth. Add salt and pepper to taste.
Put filling in a small plastic bag and cut the corner off. Pipe into the yolk wells. Sprinkle with paprika or top with a bit of fresh dill.