Are you thinking of Vaccinations rather than vitamin injections?
Ideally a kitten would have an initial set of jabs at 9 and 12 weeks, followed up by a 'booster' one year later, but if these cats were not being well cared for then they probably did not have that.
Normally the vet would discuss this issue with you when they went in for a check over or neutering.....or were they neutered when they came to you?
You could try and contact the previous owner to get the name of the vet she used and check their records that way.
If you have not registered them with your own vet yet, then I would do that now and ask for them to be given the initial course of vaccines from scratch.