I use a vaccinator gun (my sheep are on Heptavac) and the bottle stays on and goes back in the fridge (with the needle left on and covered, then new needle for next time).
For goats (Lambivac) I use individual syringe, but leave 1st needle in the bottle to draw out, then fresh needle on the syringe for each goat. Then bottle gets taped up and goes back in the fridge.
To be honest, it is in the manufacturers interest to make the bottles really single use only, otherwise they are open to being sued... so it is up to you which advice you follow. This topic is discussed on every farm forum you can find...
If you were to draw out half the bottle of vaccine into a syringe, store in your fridge for the second dose and then use other half today I cannot see how that can destroy the vaccine... but am sure others will come up with their own views...and the whole discussion will start again...