Paying the Vic

When a loan shark loans money the interest is called "Paying the Vic". What is the meaning of vic?

Not 'vic' but 'vig' --- short for 'vigorish', slang for the percentage taken by the house out of gamblers' staked money, later adapted to the loan shark meaning you cite. It seems to have originated among Yiddish speakers in New York City in the late 19th century, probably deriving from a similar-sounding Russian word.