I wish that they would make the vaccine available in smaller doses. My vet says splitting the tablet isn't a good idea because the ingredients might not be evenly mixed. I do it anyway, but it's frustrating that it has to be purchased with more doses than the average person needs.
It's only $20 from Jeffers. For the safety of your chicks I'd recommend just mixing up the full vial and putting the rest in a hazardous waste container. Both over and under dosing can have long lasting effects.
Contact the manufacturer to push for smaller dose vials. Though chances are a 5 dose vial (1 ml reconstituted) would likely run the same $20! And sold to veterinarians only since after all if we (non exterminators, non ags) can't figure out how to safely use rat bait, what would we do with needles in our hands. The 1000 dose vials are probably available to us at this time because of poultry producers not having vets available every time they hatch out chicks. And of course having good lobbyists to try and keep it that way.