The vita-mix is just a very heavy duty blender, the thermomix is a blender with a heating element plus some other function and cost $500 more. What makes the vita-mix special is the extremly heavy blades and the heavy duty moter this is what allows it to puree so well. Most other blender have weak blades and moters that can't stand up to heavy use and eventially die. The Thermomix is a multi purpose type of blender, it has a heating element that can mix foods at low or high rpm and heat at the same time, or you can use with out heat (vita-mix heats with high rpm and friction). it can also weigh ingredients. I have both and use them for different things, but if I were to choose just one I would go for the vita-mix.
