Castor oil isn't vegetable oil. It does come from a plant, but it's not digestible. This is important because the properties that make it not digestible also make it a much better engine lubricant than vegetable oil. Using vegetable oil from the grocery store won't work, partly because its smoke point is too low for use inside a combustion engine, and partly because it doesn't dissolve in alcohol, which is necessary for proper oil delivery in an alcohol-powered engine.

Castor oil from the pharmacy (sold as a laxative, again because it's not digestible) works fine, though. I've been adding a small amount (about a shot-glass-full per gallon) to my nitro to raise its oil from 11% to 12% for years.