My lab currently stores long-term in straws under liquid nitrogen, but we are considering the feasibility of storing in vials in vapour. Any info or advice would be appreciated.
I think that storage of semen in liquid nitrogen is more suitable. As sources stated there is no evidence that period of storage of semen in LN2 affected its quality. Also freezing in vials would be done according suitable protocol, it depends on volume and freezing method what you are using. For example for cryopreservation of higher volumes of bovine semen in special tubes, Harmony Freezing Method is used. When the tubes (8 or 12 ml) are frozen under detailed control of temperature and movement of samples in machine.