The vet charged you for out of date insulin?? My understanding is that Prozinc has a shelf life of 2 years, and the date on the vial is the expiration based on that. Once you actually open a vial, the manufacturer says it is only good for 60 days; however, I have heard from several people (and my vet) that it can last for several months if stored and handled properly. So, my thoughts on it are 1) if your vet charged you for that, you should get your money back, or he should give you a vial that has not expired; 2) I do not think it will hurt to use it, but you are not going to know if it is effective until you give it then test Goma in a few hours to see if her BG level has come down.
Maybe some other folks have personal experience with expired insulin and will chime in. I do not. Your vet is something else for giving you that though!