Usually when a person borrows from a "shark", the interest, or vig, or juice is due weekly, not monthly like a normal loan.

Vig is sometimes calculated in what is called points.

A point on a $10,000 loan is $100,, and is due weekly.

This does not reduce the principle it is the cost, or interest or the loan.

Also vig is what they call the money made at a sports book as well.

If a bookie knows what he doing, he can lay off half of his bets.
For instance say a bookie in NYC gets bets on the NY Yankees for the world series. Almost all his bets would be for the Yankees to win, and if they do he loses his shirt. But if he knew a bookie in Boston who had bets on the Sox, they could exchange the bets so each had 1/2 for one team and 1/2 the other.

Then, because it costs $110 to win $100, they each split, and make $10 for each $100 bet. That is also called vig. Multiply both by thousands and you have some big $$.

A loan shark, in most cases does not want his principle back, cause he can maker it up in vig real fast. Plus he is still owed the principle.