Roman numerals represent numbers, they do not represent words. The words "Vicarius Christi" are Latin, which was the language spoken by the Romans.

Vicarius filii dei

Vicarius Filii Dei is an authentic Latin papal title that means "Vicar of the Son of God." It was used recently by Pope Paul VI in Bafianae (January 11, 1968), an Apostolic Constitution elevating the Prefecture Apostolic of Bafia, Cameroon, to a Diocese. The Roman numeral values of the old Latin spelling, Vicarivs Filii Dei, add up to 666 (Rev. 13:18).

Vicar of the son of God.

You may be surprised to learn that the Latin word for "vicarious" was vicarius.