TKF,
Vertigo and Fear of heights are not one and the same thing.
Vertigo is the sensation usually described as 'the world spinning around' and as such usually stems from the inner ear which contains the semicircular canals etc which help us to maintain our balance.
Fear of heights is a different thing altogether and can give feelings of dizziness and unsteadiness on feet etc but rarely causes vertigo.
You are correct in stating that most people will only experience the fear of heights when they can reference their position to any fixed earthbound objects. A fair few jockeys on this forum have it without any problems other than some queeziness when crossing a ridge etc.
HTH
FD