Vegetable oils are oils obtained from produce such as corn or beans. Some may be all from one source; some may be blends including oils from seeds such as rapeseed or sunflower seed.
You might think that all edible oils are vegetable oils, but they’re not: there are nut oils, and there are fish oils — whale oil, for instance, was popular not so long ago.
You could technically class olive oil as a vegetable oil, but generally it is considered to be in a class by itself, and, if anything, olives are technically “fruit.”
Generally, manufacturers aim for a high-smoke point and a neutral taste to make vegetable oils as all-purpose as possible.
If a recipe calls for a vegetable oil, it generally means an unflavoured or very mild-flavoured oil such as Crisco, Canola, Corn Oil or Sunflower Seed Oil.
Literature & Lore
In 1957, the Wesson Oil company put pressure on Clementine Paddleford, America’s foremost food writer at the time, to use the phrase “vegetable oil.” Competing phrases were “all-purpose oil” and “liquid shortening.” There is the possibility that they also threatened to pull their advertising from This Week magazine if Paddleford didn’t comply. She did, after a trip down to New Orleans where they wined and dined her. 
 Alexander, Kelly and Cynthia Harris. “Hometown Appetites.” New York: Gotham Books. 2008. Page 192.