1 1-pint canning jar with lid and ring, sterilized
Wash chive blossoms by holding them upside down from the stem and plunging them into a deep bowl of cold water. Swish them around to dislodge any soil. Pour out the water and repeat 2 or 3 times.
Snip or pinch off each blossom where it meets the stem. Dry blossoms gently but thoroughly with paper towels or in a salad spinner. Pack canning jar with blossoms loosely.
Heat vinegar in a small saucepan over medium-low heat until hot but not boiling, 2 to 4 minutes. Pour hot vinegar over blossoms in canning jar, screw on lid, and store in the refrigerator until desired flavor intensity is reached, 1 to 2 weeks. Strain out blossoms and store vinegar in a sterilized glass container.