Put a small amount of fresh vac grease on the ground glass joint, and close up the vac trap. Place the trap into the Dewar flask. Most importantly, make sure the bulb with mercury in it is DOWN before opening the vacuum to the pressure guage. Periodically check the Dewar and refill with liquid nitrogen or dry ice as necessary. Continuous bleeding of argon or air into your apparatus while you are evacuating could result in liquified argon or oxygen in the trap, which could explode when the trap is removed from the liquid nitrogen.
Liquid oxygen is pale blue.