Your fuel lines will only rot if you process the vegetable oil to turn it into bio diesel. Biodiesel is technically vegetable oil, processed with methanol (the nasty corroding stuff) and some other dry chemicals in order to remove the glycerine which will in effect over time clog up your engine.
Generally speaking if you have an engine such as the 200/300tdi (not TD5 or new ford engine) with a bosch injection pump you can run your vehicle on straight veg oil without having to thin it down with diesel or process it to take out the glycerine.
Older engines like the 2.5 N/A, and 2.5TD have lucas / cav fuel pumps which wont like veg oil on its own. This is purely down to the fact that it doesn't lubricate the pump as well as standard DERV because of the extra viscosity. Thinning it down with diesel (or another chemical of your choice - illegal!) will be fine and you will be able to start on it.
The annoying thing is that the stuff has gone right up in price from when I started. I only used to pay 53p a litre, now its within the region of 99p to £1.10 depending on whether you want to run a bit of crisp'n'dry through your engine (no difference what so ever), so its not as worthwhile (cost wise anyway) as it used to be.
I have just found a way (perhaps) to get hold of it for less than 50p per litre where I work for pocket money. I'm going to ask Head Chef next time I see him, so I may be stinking the street out again!
-Pos