Vegetable oil is used for transportation in 3 different ways:

Vegetable oil blends:

Mixing vegetable oil with diesel lets users get some of the advantages of burning vegetable oil and is often done with no modification to the vehicle. 

Biodiesel:

If vegetable oil is transesterified it becomes biodiesel. Biodiesel burns like normal diesel and works fine in any diesel engine. The name just indicates that the fuel came from vegetable oil.

Straight vegetable oil: 

Straight vegetable oil works in diesel engines if it is heated first. Some diesel engines already heat their fuel, others need a small electric heater on the fuel line. How well it works depends on the heating system, the engine, the type of vegetable oil (thinner is easier), and the climate (warmer is easier). Some data is available on results users are seeing. ] As vegetable oil has gotten more popular as a fuel, engines are being designed to handle it better. The Elsbett engine is designed to run on straight vegetable oil. However, as of the start of 2007, it seems that there are not any production vehicles warrantied for burning straight vegetable oil. Also there is no specification for what fuel-grade vegetable oil is (things like maximum-water-content, energy content, etc). At this point straight vegetable oil is only a niche or experimental market.

So....The transition and the trend starts with....Biodiesel and Vegetable Oil Blends.

Source: "Vegetable Oil Economy", Selected Articles: www.filterinternational.com