Hept vac will keep but you must buy the proper applicator for it, when we got oursthrough our vet, you puncture the rubber top and the bottle stays in position, the syringe is 2ml and it is very similar to a drench gun for cattle, inject, squeeze the handle and when released the syringe fill up again, keep it in the fridge till next needed, take out a couple of hours before needed the next time to get it at room temp and do your second dose,this is then when the remainder of the medicine can be disgarded, medicine companies don't tell us to do it the way i have described above, they are only wanting more money from us, the bottle is air tight but remember do not take the bottle from the applicator, no need then to keep pricking it with the needle.
HTH