Dear Donna,
visual dependency means that one's brain relies too much on visual input (as opposed to other sensory input). when visual input becomes unreliable for whatever reason - it can for some people during migraine - we need to rely on vestibular input that much more. those with visual dependency have trouble doing this and the result is dizziness. although i don't know because i am not a doctor, what you describe does sound consistent with visual dependency, as does the therapy you are being given - training the brain to rely on vestibular signals more.
i didn't know they could actually treat this... i am glad for you. i don't get dizzy normally, and only very occasionally during migraine, but it is a feeling that i despise when it happens. i can't for instance handle spinning around or doing any sorts of rotational type coaster rides. some people love this stuff - not me - then i do get dizzy very quickly, and feel sick blechh.
good luck with the treatment i hope it helps soon!
~ waves ~