I believe that the Lantus is delicate in some way.

Using past the 28 days MAY work, and it may not.

Here is a link that illustrates that:
http://care.diabetesjournals.org/content/26/9/2665.full

This may be due to how the Lantus is synthesized. And it may be more prone to bacterial degradation, from repeated needle withdrawals.

Drug companies do extensive testing on products with short expiration dates. So I would think in this case they found that Lantus just does not keep. Some of that is overkill, perhaps, and the only way to know for sure is by checking your blood sugars at the beginning of a new vial, and at the end of a vial to see if they change like that article I found for you.

For a mild diabetic like you are Mel, it may not be as critical as in a brittle diabetic using higher doses. You'll just have to test it out yourself. I see some people online using it up to 45 days with extreme care as to not contaminate the vial.

One thing I would try is keeping the vial in a ziploc in the frig. And even some light blocking type of container. A large amber drug vial may be sufficient.
Or some other container, so airborne food nano particles cannot get to the top of the vial and also protect from light. Bacteria, fungus spores can then settle on the lids of insulin, so keeping the open vial as clean as possible would prolong the shelf life once opened. And notice the light sensitivity! Protect from light. For example, I would not warm up the whole vial prior to injecting. I'd pull up your dose, put the vial back in the frig away from light and let the syringe warm up a bit.

So the bottom line is ???? you may or may not be able to squeeze more days out of your expensive Lantus.