[pharnorth]

If you are looking to vaccinate and are somewhat scientifically minded you might like to read the guideline on responsible use of vaccines by Ruma, on www.Ruma.org.uk as this will help with any thoughts on sharing/ reuse of the vaccine.

I spent many years in drug development though have no direct involvement with Hepvac I have many years experience setting shelf-life and in use periods for drugs. It is highly regulated so what appears in the instructions is based on proof that the vaccine is effective if the instructions are followed. Companies always want the longest shelf-life and in- use period as it reduces supply chain costs and gives them an edge over competitor products. 

The 10 hours in use limit is most likely primarily down to oxygen as these are anaerobic pathogens so the more you take out the more air will have displaced the headspace which was most likely flushed with Nitrogen on manufacture.  The effect would likely be loss of efficacy. Generally in use periods for vet vaccinations will be short because of potential contamination also.   So safe bet, follow the instructions.  I can't see cling film will help much because once you draw out a few doses the reduced volume will be replaced with air.  Personally, I might wrap it in cling film for over night if sharing with a second flock or I didn't complete the job on day one but wouldn't risk a month.