Good news is, you are over thinking this one brother. All you need to do is draw oil from one vial, then go to the next. Of course remembering safe, sterile practices. If you get 1/2 way through a vial and felt you wanted to, you can always vent it by sticking a pin without a syringe through the stopper. Easy enough. Put it this way, Many of my clients use more than one compound at once and this is a topic/issue that has never come up before.