Advice can be a dangerous thing, so I'll try to explain my credentials:
My company in Rotterdam builds process analysers and robotics systems for the petrochemical industry, and in my spare time I'm secretary of an Institute of Petroleum technical committee, and the UK representative to CEN on technical aspects of motor vehicle fuels.
In summary:
1. Most diesel fuel in Europe is (or will be) a 5% blend of bio-diesel in petroleum diesel. All the car manufacturers have accepted this sitution. and the bio-diesel content will continue to increase.
2. Bio-diesel and diesel can be used in any percentage combination without modification to a modern diesel engine, but the car manufacturers have only validated long term operation with 5% levels (important if you car is under warranty)
3. Bio-diesel is not vegetable oil, but is made from veg oil.
4. A hot diesel engine will run on 100% veg oil, but read item 6. before you try this.
5. A cold diesel engine will not start on 100% veg oil, but a blend of 25% veg oil and 75% diesel will start, probably upto 50% 50% if you want to experiment on a hot day, but read the following first.
6. There is a real issue with straight veg oil that will cause premature engine failure. Veg oil is a chemical compound made up from one molecule of glycerol and three molecules of "fatty acid".
When you burn veg oil in a diesel engine, the fatty acids burn but the Glycerol does not completely combust and forms gummy residues in the injectors, cylinder head and around the exhaust valves. This means that eventually the engine will need a visit to the workshop, and consequently if you use veg oil in a diesel car this invalidates the warranty.
If you look at a used chip-pan, or even an old bottle of oil in the kitchen, the gummy (brown) resdiues you see come from the glycerol in the oil.
Bio-diesel is a processed form of veg oil, where it is heated with methyl alcohol and a catalyst, creating "fatty acid methyl esters" or FAME. FAME is bio-diesel. During the chemcial reaction, the glycerol splits off as a separate layer - like a bottle of oil and water - and the waste glycerol is run off and used in other products.
Summary:
1. There is a tax issue where you need to pay H.M. Customs if you add veg oil
2. There is an engine reliabilty issue with the use of veg oil - unburnt glycerol gummy residues in the engine
3. If you car is outside warranty and you can do the maintenenace yourself, then most days a 25% veg oil mix with diesel will start OK. In winter (freezing) a 10% mix should be OK
Robert