We have examined the deformation of vesicles on surface topography. From known vesicle shape dynamics, we expect temperature to play a major role, however, when cooling vesicles in the presence of a low-permeability solute, the mechanism of deflation is not clear. We investigate giant unilamellar vesicles on topography to quantify their cooling over a range of temperatures. The volume constraints point to a transient defect pore model as being responsible for vesicle deflation over the temperature range studied.