Veggie oil in diesel engines???

[baggygreen]

My olds recently bought a 2.0 TDI Jetta, (took it for a spin last time i visited, runs like a beauty, too!)

But i dont see them being happy if I were to suggest throwing vegetable oil into the tank. Id want to see substantive proof from VW that yes, vegie oil will run perfectly well and not harm the engine before i even suggested it to them.