OK, after some research on the internet (which I think is great, try it out sometime) I found out that 'essere' needs the preposition 'di' and 'venire' needs 'da'.
Furthermore, 'Di dove sei?' asks for your origin in the meaning of a characteristic, like 'Where are you from?' - 'I am from Wonderland.' (this means I am wonderlandish as a characteristic) It is being used in place of the english genitive, as well.
'Da dove viene?' on the other hand is asking where your movement has its origin, 'Where do you come from?' - 'I come from school / I come from work / I come from the shop.'
Hope that helps and is comprehensible.
Unfortunately, "of where are you from" doesn't mean anything in English. English speakers would not understand what you are asking if you said this, except by guessing that mean "Where are you from".
"from where do you come" makes sense, but in idiomatic English it would be "where do you come from".