I have often looked at terragen and i do like some of the pics it produces, however, I do think you can get simaler results with vue - with some trial and error. How ever i agree with the many people who have asked why you'd want too!
Vue is amazing in that it can produce a huge diversity of landscapes, however IMHO all depends on the lighting and atmospheres, you can have created the most realistic land scape possible in vue and then find that the lighting does not work at all. However there are a huge number of rescorces avalble to help you here, and at geek at play, and of course the forums and gallerys at cornucopia.
Placement of rocks and plants etc is some times trickey, especialy rocks, unless they are mixed with other vegitation , they should always be buried in the surface, if you look at the average rocky landscape then try to dig one of those rocks up, youll probhably find that like the ice berg, up to two thirds of it are under the surface!!
But i would strongly suggest that you dont try to reproduce a terragen landscape in vue, you wont, its not terragen and does not have the same look, simple as that, and its really much more fun and far more productive to look at what vue does well and use those strengths such as the material editor and function editors which can get you practicly what ever you want with the right knowlege and some trial and error!!
Ive also played with geo controll and might well buy it!! Its a great way of creating your own custom terrain maps to import in to vue.