Vision tests for blindness are always difficult but I would be concerned about nightblindness causing strain. Vision may be reduced during the night but fine during the day. Tests that can be carried out would entail an obstacle in light and then in a dimmed environment or involved dropping a cotton ball in front of the eye to watch the movement etc.
To get a better idea of the actual integrity of vision and health of the eye, a full opthalmoscopic exam including visualising the structures of the back of the eye - the retina and optic nerve, would be required.
It is not a secret that most veterinarians have a weak point with eyes since the training of the eye at the universities is really not comprehensive enough. You may want to find a Veterinarian that is more experienced with eyes, or you may want to express concern to your Veterinarian and ask her for a referral to an Ophthalmologist.
Martin Slome DVM
Centre Street Animal Hospital
7700 Bathurst Street
Tel: (905) 771-9855