Final answer:
The formula for vanadium (III) phosphide is VP, which results from the +3 charge of vanadium (III) and the -3 charge of the phosphide ion.
Step-by-step explanation:
The formula for vanadium (III) phosphide is established by balancing the charges of the vanadium cation and the phosphide anion. Vanadium (III) indicates that vanadium has a +3 oxidation state, and since phosphorus in phosphides has an oxidation state of -3, we can deduce that the formula should be VP, where one vanadium ion balances one phosphide ion.