the Bosch pump in the Tdi engine is Ok with veg oil - direct injection engines (which the Tdi engines are) have the problem that cold vegetable oil can get sprayed past the piston rings and contaminate the sump oil. You should be ok with 50:50 diesel:veg oil in summer as long as you regularly change the engine oil. Any more than 50:50 and its worth fitting a twin tank fuel system where you start and finish a run on diesel and switch over to veg oil once the engines warm
Hope this helps - just fitted a twin tank system to my discovery, seems fine so far!
Ed.