If you draw back the syringe to the amount you are going to draw and pressurize the vial it is a bit easier to make the draw.
This has the effect of contaminating the vial with the air in the syringe which I conclude is less of an issue than drawing the air from outside the vial. Pressures have to equalize or there would be a vacuum.
This could be one of those things that most people other than me know.
Where can I get reputable b12 vials and syringes?
b12 vials multiuse 
Problems purchasing B12 vials
Vial of 30 ml versus single ampoules 
