We introduce a new method to measure the energetics and range of weak biochemical bonds using functionalized vesicles. Large bilayer regions are held in molecular proximity by osmotic depletion forces to enable rapid specific bonding. By fixing an electrical charge to the tethering site of the functional group on one surface, persistent adhesion of the vesicles after removal of the depletion stress is titrated against the clamped electrostatic potential of the opposite surface. We demonstrate the method with DNA bases and obtain new information on the range of their specific interactions.