"Da dove vieni?" and "Di dove sei?" have more or less the same meaning. They're both used to ask where one persone comes from. Altough there's a slight difference between the two:
1. To answer the question "Da dove vieni?", you should answer mentioning the name of your city or your country. Es: Vengo da Firenze / Vengo dalla Toscana;
2. To answer the question "Di dove sei?", you should use the so-called adjectives of nationality. In fact, the verb "sei" = essere indicates a physical or a personality characteristic of the person. Es: Sono russa / Sono italiana --- but also Sono bionda / Sono alta.
Finally, pay attention not to mistake the verb VENIRE with the verb ANDARE: they have a similar meaning but their use is different.
Spero di essere stata utile :)