The two tank systems are the best, and really only option if you want your car to last. The Idea is that you start your car on regular diesel and run it on diesel until the veggi tank has been sufficiently heated. At this point the car is switched to vegi. When you decide to turn the car off you once again switch to diesel to purge all the vegi out of the injection pump so that no cold vegi is in the pump on the next start up.
Even with all this however it is still not particularly good for the engine. You'd be much better investing your time in money in making or buying a setup to produce biodiesel from your vegi oil.