As I know, in the Vishnu Purana the royal line of Anga is narrated, but it refers to Vrishasena (eldest son of Karna) as King of Anga.
Why does Vishnu Purana mean so although there is no reference in Mahabharata for such a thing?
Why not? There are many incidents only mentioned in Puranas and not in Mahabharata. It is the characteristic of Puranas to detail about lineage of great kings.
O brāhmaṇa, authorities on the matter understand a Purāṇa to contain ten characteristic topics: the creation of this universe, the subsequent creation of worlds and beings, the maintenance of all living beings, their sustenance, the rule of various Manus, the dynasties of great kings, the activities of such kings, annihilation, motivation and the supreme shelter. Other scholars state that the great Purāṇas deal with these ten topics, while lesser Purāṇas may deal with five . [Srimad Bhagavatam 12.7.9-10]
Mahabharata is a basically the story of Kuru dynasty and not other dynasties. That is the reason why we do not find detailed stories of Krishna and his dynasty. We only find mention of his ancestors.
Another example is history of Vijaya bow. In Mahabharata, Karna only mentions the tradition the bow is handed to him but Bhargava Charitam of Brahmanda Purana gives detail how he earned it through penance.
Hence, we don't find mention of detailed stories of Karna, Drona and other important characters. It is Purana's nature to mention the history of dynasties like Solar, Lunar and other dynasties.