A word of caution: disease transmission.
It's one thing to use a multi-use syringe - and, I'm presuming, multiple-use needles - on stocker and feedlot cattle that are going to have a pretty limited lifespan - but, on a breeding cow herd, I wouldn't be inclined to use one.
But if you must, and If you have any concern (and you should) about transmission of blood-borne diseases like anaplasmosis and bovine leukosis virus, you'd be well-served to consider switching needles between each and every animal - and no 'in-and-out' of vaccine bottles with a needle that's been in an animal.
Yeah, it's a royal pain in the butt to have to do it, but disposable needles are pretty cheap, compared to losing one or several cows to anaplasmosis or BLV-induced lymphosarcoma, etc.