I never vent when I do this. What I do is put a little in the new vial, say 2ml, then draw up 2 ml of air. This air comes from the vial and goes to the top of the barrel. I then repeat this until I have all the gear in the vial and the same amount of air in the barrel of the syringe. No problem at all. I do this when I'm stacking two or more oil based roids. I know what percentage of each I want so I make it up ahead of time in one vial. shake it before drawing and I'm done.