Yes, TIS, vig is short for vigorish. It is interest stacked onto the principal (the principal being the money owed in the first place).

Example: Joe Gambler goes to the local bookies, makes a bet at a card game. Loses. The guy now owes Don Bookie money. But Joe can't pay Don the money--'cause he simply doesn't have it.

Don introduces Joe to a good friend--Larry Loanshark. Loansharking goes hand-in-hand with gambling. Larry offers Joe money--hey, don't worry about it, it's my favour. Joe, of course, has to accept, as if he doesn't then he'll find Don Bookie paying him a visit. The trouble is, Larry charges interest--say, 10%. This is the "vig." Joe doesn't pay the first week. Fine. Larry tells him he'll get it next week. But the vig is added to the principal--Larry needs his interest, after all--and so the 10% of the new price is going to be higher than last week, so the money is going up each time. Basically, Joe is f***ed, and there's no way out of it for him.

Mick