Meclizine can be diagnostic. I was looking through a presentation of Dr.Hain. He clearly states Mecilize will not work for MAV/VM as it is a vestibular suppresant. I think it can diagnostic in the sense if meclizine works for you it is time you and your doctor took a closer look at the ear to see what is wrong.Migraine is central and meclizine will not touch it.
EDIT: The same cannot be said of benzos as they affect a lot of neurotransmitters and are a vestibular suppresant. So benzos will work irrespective of the cause being ear or brain. Thought people might appreciate this small nuance.