What are Vergil's credentials for the job?

THis is a pretty good description of Virgil. You can check the rest out at the source-link below.

Although Virgil’s official job title is a "guide" for Dante, we all know there is more going on. Virgil quickly goes from tour guide to personal tutor, liaison, and father figure to Dante. Even so, Virgil does a tremendous job as the tour guide. In each canto, he does some straight-up lecturing, but Virgil does show-and-tell too, with real-life sinners. He’s quite a speaker, able to convey to Dante such huge concepts as the role of Fortune in the human realm, the internal structure of Hell, the origin of the Underworld rivers, the geography of the earth, and the Harrowing of Hell.