Step 1Trim the stem ends of the haricots verts and cut the beans into 1-inch lengths.
Step 2Bring a 2 1/2 -quart pot of water with 1 tablespoon salt to a rolling boil. Blanch the beans until just cooked through, 2 to 4 minutes depending on thickness and age. Drain immediately and transfer to a bowl of ice water to chill. Drain and pat dry.
Step 3In small bowl set over the larger bowl of ice, whisk the cream just until it thickens and the whisk leaves a trail in bowl, 1 to 2 minutes. Using the whisk, gently fold in the vinegar, about one-fourth teaspoon salt and pepper to taste; do not overbeat.
Step 4Toss the beans with cream, garnish with chives and serve at once on individual plates or a platter rimmed with halved grape tomatoes.