In order for Vegetable oil to cause coking on the cylinder walls or piston rings the oil would have to make it into the cylinder and make contact with these parts.
That is almost impossible with our indirect ignition engines. It would be more of a factor with an direct injected engine where you would have to blend to 50-50 or more.
Some people run on straight oil. Personally I won't do it. You want to mimic the viscosity of diesel oil as best you can. To do this you blend with RUG or Kerosene. You don't want to blend with diesel because you want something of a lower viscosity to thin out the veg oil.
Some people have reported problems blending with Diesel fuel. This happens with the additives in diesel reacting to undesolved fats in the oil. This happens usually with improper filtering or cold weather without a heated tank.
And last, take "opinions" with a grain of salt. Do your own research and draw your own conclusions. You will find better information here:
http://biodiesel.infopop.cc/groupee
Danny