If it's an unused - meaning you haven't been in & out of it with a needle - and it's been properly refrigerated since you got it, it's *probably* OK, and I wouldn't hesitate to use it on my own cows - but I wouldn't sell it to a client, and while I can't tell you with all certainty that it's still effective - but I'd bet you a Coke it'd still be good.
That said, if you're buying more, and just need this bottle just to finish up a few head, hold it 'til the end of the herdwork, and make note of which cows/calves you vaccinate with it, so that if you encounter any problems, you'll know better next time.
Additionally, by vaccinating your herd, you're not necessarily going to 'immunize' every single animal - but if you can get 65-70%(or more) of them successfully immunized, then the chances are that if a pathogen(virus, bacteria, etc.) is introduced to the herd, it'll more than likely encounter an immunized animal and not get a chance to amplify or spread.