There's no Dewar vessel on the LV as far as I'm aware of. The surface of the stage is the propellant tank, made of simple Al-Li alloy, several millimeters thick. It works fine because of the large mass of the LOX and the comparatively short time of the stage's operation. You just can't transfer enough heat into the stage naturally in the short twenty or thirty minutes between fueling and launch to cause any trouble.
Before they started supercooling the LOX, it was even easier because heat transfer into LOX tanks is conventionally managed by boil-off (in launchers without supercooled propellants, latent heat keeps the liquid at a stable temperature and a trickle refills the tank continuously until a minute or two before launch).
Before they started supercooling the LOX, it was even easier because heat transfer into LOX tanks is conventionally managed by boil-off (in launchers without supercooled propellants, latent heat keeps the liquid at a stable temperature and a trickle refills the tank continuously until a minute or two before launch).