This is an interesting point to bring up, and it may bring some things to light about insulin degradation, and some of the possible causes.
We don’t really “open” an insulin vial ever. We take the cap off and start using it, but that does not open it. The insulin is still enclosed with the rubber stopper, and the stoppers are self-sealing.
After we take some insulin out, it is still closed inside the vial by the stopper.
The only difference when we start using a vial is that we are introducing outside air, and also exposing the insulin to more surface area of air.
A bit hard to describe with words, so here is a quick illustration I made to show what I mean.
This image is assuming someone is storing their vials upright, but the same applies no matter how you store your vials.
As @Finn mentioned cartridges, the difference there is that the insulin is NOT ever exposed to air. Since the plunger moves, there is minimal air contact.
Another image, although this is pretty obvious to everyone already. But I just like adding the images. 
What I do is minimize air exposure in the vial by keeping it under negative vacuum.
When I first start the vial, I take out 200 units of air right away. That starts it at negative vacuum.
Then, I always remove less air than insulin I am taking out. That keeps it at negative vacuum.
For example, if I am removing 100 units of insulin to put in a pod, I only put in about 65 units of air.
Thoughts on this?
BTW, if you do a google search on the phrase negative vacuum in insulin vial, guess what the first link is?
Yay! It’s FUD!
This post describes it pretty well:
@jbowler, this seems like something you would have thoughts on too, so adding you.