The process of turning vegetable oil into a diesel fuels substitute (biodiesel) is a bit complicated, but is only required when you want to run 100% vegetable oil based fuels. Some vehicles, as has been noted, can run straight vegetable oil. I would have zero concerns (and I am a naturally paranoid type of person) dumping a half gallon to a gallon of vegetable oil in a tank of diesel in my f350. I would have even less concerns running up to 10:1 diesel:wesson in my kubota. Its fuel injector system is a heck of a lot less complicated than the one in my truck.
Don't waste it, just toss it in the fuel tank and burn it. Though, I will agree, don't do it in the winter or when temperatures are routinely coolish..