Vergil did not start the tradition that Aeneas was the ancestor of Romulus...that had been around for a long time. Julius Caesar minted coins with Aeneas carrying his father Anchises on his back...the Julii claimed to be descended from Iulus, a son of Aeneas.
A number of vases with Aeneas on them were found at the Etruscan city of Vulci, as if there was a strong interest in him there. Maybe the Romans picked it up from the Etruscans.