Vets are useful to help speed up the natural recovery process, helping your horses regain their physical and medical condition after a big race or a taxing training session.
Again, just like Trainers, before you can give your horses the veterinary attention they need, you'll need to first have some spare Vet capacity. You can hire Vet Services (an hour at a time) to come and work with your stable on the '<a href="facilities.php" class="action">Stable facilities and training</a>' page. Unlike trainers though, any excess time that the Vet has while at your stable will not be sold on for use by the public - so only hire the vet for as many hours as you think you will need!
Once you have some spare vet capacity, it can be assigned in blocks of one hour to your horses on the 'Stable facilities and training' page. This page will also show you the time and date that the vet is going to come visiting. As you might expect, this time and date is also recorded in the horses' Upcoming Schedule. Once a vet schedule has been successfully arranged, the vet will continue to come and see your horse/s for the same amount of time each day in the vets' working week - not stopping until you either make changes to the schedule, or until the horse fails to keep a veterinary appointment.
If you have the Enhanced Vet Centre Specialist Facility, then during the nightly Scheduling process (approx 00:00), any unused vet capacity is automatically assigned to the horses in the worst medical condition.
Remember that the horse will be busy with the vet for however many hours you have assigned, so be sure the horse isn't booked with the vet for so long that it misses out on something else like Race Preparation!