Q: Can the same bottle can be used for the initial doses and for the boosters a month later, if kept refrigerated (I know people do this, but is the second dose still effective?).
A: No guarantees, but should be fine. What you would be best to do is draw 20mls {for my 10 sheep} from the bottle with a clean syringe and put in a clean dry container/bottle then draw your 2mls from there for the initial dose. cover the top of the bottle with some sellotape and refrigerate.
Womble, I am afraid to say that your vet is taking a risk (professionally, if nothing else, as a client could hold them to what they said in case anything goes wrong). Yes, it's true there's no guarantee one way or another and probably won't do much harm to re-use the bottle but nobody knows how much effectiveness would remain and as I said before, it's probably just going to go off and be useless. And there is of course the risk of introducing bacteria in the bottle when you take out the first doses, potentially inoculating disease yourself.
By all means anyone is free to do what they want with their animals but don't go crying to the vaccine manufacturer's if the product is used off label and then it doesn't work or worse, causes harm.