I used Heptavac-P on the moorland farm. Yes it says use the same day but we did use it after longer than this - typically for the booster when doing the initial course. I don't think I'd rely on it much over a month to six weeks later, though. I'd be dubious not using it up till March.
That's bad news if you have 25 doses for 2 goats.
Why did the vet stop selling you just a couple of doses?
Maybe a small-scale neighbour could share with you? Or another time, would a local farmer let you have the end of his/her bottle when his/her flock is done?