This is something that intrigues me, since according to Vaishampayana, Karna was called "Vaikartana Karna" after donating his armor to Indra (which happens in Vana Parva) but in Ranbhoomi (battle field) he is called Karna, and not Vasushena (his name of birth).
So, my question is, why was Karna called with such a name if he still did not donate his armor? Does the donation of his armour happen in Adi Parva? It is not a question of his other names, it is a question about why it is so called if it received the name "Vaikartana Karna" after the donation of its armor.