Some vets (like several mentioned here) fully support it and encourage their clients to do their own. Others, however, feel like you don't need their help if you don't have them out for your annual visits. I've known some vets that drop their clients if they do their own vaccines! There are a few reasons for this. 1) Vaccines have a large markup and are very quick and easy to give. A vaccination visit can net hundreds (or thousands, in a larger barn) of dollars in a half hour's work. 2) The vet gets to check up on their clients, see the horses, evaluate feed and condition, and discuss any chronic conditions that may be present. 3) If the vet doesn't feel as though you are an active client, they see no need to jump out of bed at 2am to service an emergency. I've also known people who started doing their own vet work, and when they DID need a vet for a colic or such, everyone they called turned them down, saying they only service their clients after hours, and they should contact their primary vet. Primary vet said "who are you? I haven't been to your barn in years, contact your primary vet". SOL.
I ALWAYS encourage people who want to do their own vaccines to talk to their vet first. Some vets will be very supportive. Some vets know their clients and know its not in their horses' best interest to have their owners vaccinate them. Other vets will want their business and tell them no. Other vets will sell them the vaccines at a discounted rate and not charge for administration, so both parties win. Plus, someone needs to teach the horse owners how to give a safe injection. Where, how to pull back, what to do if blood is hit, how deep, what dose, etc etc etc. Definitely talk to your vet before just jumping in!