It's simple math. Consume less than you burn.
This idea is simple, pervasive, and wrong. The body's metabolism responds dynamically to the amount of fuel available, and exhortations to "eat less" are not at all helpful to someone who is trying to lose weight.
Machines require proper fuel, and proper lubricants. Vegetable oil is NOT a proper fuel for warm-blooded bodies. Polyunsaturated vegetable oils suppress the metabolism, shunt carbohydrates to fat production, are not easily burned for energy, and distort the body's hunger signals.
Using vegetable oil in the human machine is like substituting a random grade of mineral oil for the transmission fluid the manufacturer designed your car to use. Your transmission might work for a while, but I wouldn't expect it to last for long.