I wouldn't worry about the amount in the vial, there will be a 'fudge' factor in there to ensure 5 complete doses.
It's very normal for vaccines to be diluted with saline (pre-mixed shelf stable ones in syringes have them too), but the saline is to reconstitute freeze dried vaccines, and also to meet the requirements for injection volume, sterility, pH, chemical balance, etc.
There are strict labelling controls on the boxes on how to use and then also,in this case, I know there will be good training on how to mix and administer the vaccines.
These vaccines themselves aren't too unique for administering, it's the cold chain storage and cyber security around the tracking of all this that are the main concerns.
I'm going to an IMechE webinar on Friday to explain more, with all kinds of experts in the field. Should be interesting!