A thermodynamic system: Sublimation of ice from a glass vial

The vapour (generated from ice sublimation) is accommodated within the head space of the vial by pushing back the atmospheric gases of the partial vacuum, in order to make room for itself. 

To visualise this process, imagine an infinitely thin (weight-less) and friction-less piston, which fits inside the vial and is pushed upwards by the newly generated vapour. The boundary defined by the piston then allows for the definition of 'the system', as the ice mass and the new volume of gaseous water molecules, with everything outside of the system being considered as 'the surroundings', i.e., any remaining head space inside the vial, to the volume of space within the drying chamber, to the volume of space inside the condensing chamber, and beyond.