There was no bishop in Vasto at that time, and therefore Celestine, the bishop of some other place nearby, is asked by Gelasius to ordain Julian as the presbyter, but not as a bishop, because Celestine acts only as a visitator, administering the Church of Vasto until the new bishop is elected (see PCBE Italie 1: Iulianus 13).
It is unclear what the "cardinal priest" means here. The title "cardinalis" is usually used to describe the parish priests of the city (at least in Rome), but the word "pontifex" used here seems to suggest a bishop.