Mix together beans, radicchio, 1 tbsp. vinegar, olive oil, chives, salt, and pepper in a small bowl. Taste and add more vinegar if you like.
Trim base from endive heads and separate endive into 24 leaves, reserving smaller leaves for another use (like salad). Spoon bean mixture into base of each leaf, dividing evenly.
Make ahead: Prepare through step 1 and chill, covered, up to 1 day.
Note: Nutritional analysis is per spear.