As far as I can see, the price of vegetable oil lags the price of diesel only slightly. It's only in the few weeks after a sharp increase in the price of diesel that it's genuinely worth doing.
If you have access to free oil from a chip shop or kebab house, that's a totally different matter. Getting kit to convert the oil to bio-diesel and properly convert the car will quickly pay off.
I believe the limit is 2500 litres per annum, which even if you can't hit the kind of MPG a JTD would get from a forecourt grade fuel should still be good for 20k a year or more.