Please please please don't give your own vaccinations!!! Unless you have worked as a vet tech or been trained by your veterinarian, please leave it to the pros...
1) If you vaccinate a client animal you are liable for malpractice if something goes wrong.
2) If your horse reacts to the shot you may not be able to save them, whereas a vet can
3) If you give vaccines yourself and something goes wrong, your insurance will not cover it
4) Your vet makes the main portion of their income from shots... so if you take your herd off their list then want their services, they may not be interested.
5) You still need a vet to administer Rabies and EVA vaccines. While you can technically give either, the state does not recignize them unless a vet has given them. That means that if your herd is exposed to rabies, your reciept will not save them from being killed and tested, even though they were vaccinated. And the same with EVA... your horses will be branded (not literally) as EVA positive even though you know they are just positive from the vaccine.
If you do want to give your own shots, make sure you have your veterinarian's full approval and have them train you properly. They can describe the risks (however minute they are) and teach you how to address things. And if you have a very good relationship they can cover your actions under their insurance, to avoid voiding your mortality insurance.
All that said it is a great way to save some money and some stress, but you have to calculate the risks carefully. I do my own shots, knowing the risks.