Why is the product of vegetable oil and methanol a better fuel than vegetable oil itself?
What you're basically referring to is biodiesel vs. straight vegetable oil.
A diesel engine can be fuelled with straight vegetable oil, but there are a few disadvantages.
1: Straight vegetable oil tends to be higher viscosity than biodiesel which makes it more difficult for injection pumps to handle it.
2: Straight vegetable oil often has rather poor low temperature properties. Especially at sub zero temperatures, where it can turn somewhat waxy, or at least very viscous. Biodiesel suffers less in this regard, especially if supported by a pour point depressant.
3: Some straight vegetable oils (especially those with a degree of unsaturation, can cross link on exposure to air. This is particularly noticeable when the oil is present in a thin film. It is undesirable to have a surface coating of non-soluble cross linked veg oil on the inside surface of engines.
There's 3 pretty good reasons anyway.