The 28 days is for an unrefridgerated vial. Keeping it in the fridge and taking care to not roll or shake it, you can get it to last quite a bit longer.
Our vial lasted 2.5 months. It probably could have gone longer, but I kept thinking I was catching glimpses of floaties and the numbers were flatter, not as much dip, and I didn't want to jeopardize going OTJ so I switched. I see many people use their vials for longer than 28 days, you just need to watch the BG numbers. Some kitties are more sensitive and their numbers will start to creep up as the Lantus gets a bit older, some can go on the same vial for quite a while.