The engine has to be hot and the oil also needs to be warm so you start your car on diesel or biodiesel, the car switches to vegetable oil when temperature sensors tell the controller the right temperatures have been reached and a few minutes before reaching your destination, you press the flush engine button to switch back to diesel or biodiesel then you idle the engine till the flushed light comes on before turning the engine off. If you turn your engine off while not flushed, a warning buzzer sounds to tell you to turn the engine back on. If the engine cools while oil is still in the engine, the oil thickens and polymerizes effectively turning into plastic.
The conversion kits often include either an electric heater in the vegetable oil tank or an engine coolant jacket around the vegetable oil tank ( keep in mind that it takes diesel engines a long time to warm up in winter ). In cold areas, you may also want the vegetable oil lines insulated and or heated. One manufacturer has a 2400 watt heater that you can plug into a wall socket to rapidly heat up your oil before leaving the garage ( I suppose you could fry some french fries too ).
Diluting vegetable oil with a thinner is not a good biodiesel, real biodiesels are when the oil molecule has been broken into esters, the esters have fewer carbon atom in the chain and are more linear, it's not just a matter of thinning out the fuel but some people do mix their oil with a thinner.