I suffer from vertigo when I am on a tall structure and exposed to the elements. I can look down from the top of a tower so long as there is a nice solid railing (that I've already 'test-pushed'!) I can rest my arms on, but never lean against!
I think that it is the feeling of being 'exposed' that causes most fears of heights. I don't mind being in high or low wing a/c equally, but it is because in both aircraft I feel enclosed and safe.
In some ways, I find it worse to look down from a height of 100-500 feet than from, say, 2,000 feet upwards, because it becomes more difficult to reference ground objects above about 500 feet.
I don't know, all in all, I feel safe in aeroplanes and don't suffer any height related fears
