I'm not a gambler, (and I don't understand the "vig" reference) but I believe the point spread is intended to help even-out the betting for each team. As the bets heavily favor one team, the spread will change to offset that disparity. It prevents them from going broke on an upset. The "over/under" on the other hand is based much more on each teams ability to score and/or defend. It tends to change more with injuries or anticipated weather than with the betting trend. Many folks tend to bet "one way" all the time. My brother in laws favorite expression is "you're never out of it when you take the over". He's now broke...