In Configuration > Medical > Staffing you can add staff veterinarians and staff surgeons to your shelter's vet lists. Vets can add their signatures here using the mouse or touchscreen (they will need to have both the "Configure My Shelter's Medical Module" permission and the '"Configure My Shelter and its Users"' permission in order to save their signatures).
The veterinarian signatures will populate on the system generated Rabies Certificates and the surgeon signatures will will populate on the system generated Spay/Neuter Certificates.
Please keep in mind, deleting a vet signature that is no longer working with your organization will remove the signature from displaying on historical system-generated documents such as Rabies or Spay/Neuter Certificates when accessing these from an animal's record.