WVO is a short way to say waste vegetable oil.

"Many enthusiasts refer to vegetable oil used as fuel as waste vegetable oil (WVO) if it is oil that was discarded from a restaurant. [...]

As of 2000, the United States was producing in excess of 11 billion liters (2.9 billion U.S. gallons) of waste vegetable oil annually, mainly from industrial deep fryers in potato processing plants, snack food factories and fast food restaurants. If all those 11 billion liters could be collected and used to replace the energetically equivalent amount of petroleum (an ideal case), almost 1% of US oil consumption could be offset."

http://en.wikipedia.org/wiki/WVO