The best explanation for the difference in the names of PCl3 (Phosphorus trichloride) and VCl3 (Vanadium (III) chloride) is:
Vanadium is a transition metal with multiple ions so we use the Roman numeral in the name for the ionic salt while phosphorus and chlorine are both nonmetals so we use the prefix system to name covalent molecules.
This statement accurately reflects the naming conventions: transition metals can have multiple oxidation states, thus requiring Roman numerals to indicate their specific charge in the compound. In contrast, PCl3, being a covalent compound formed from two nonmetals, is named using prefixes to denote the number of atoms present.