Lighting, materials, and more lighting. 

A straight un-tweeked Vue render has 100% hard, dark shadows.  No matter what you do, unless you adjust the lighting, it will always look like a 3D render.  It can look good, but it will never look real unless you really work on the lighting. 

Same with the materials, too much bump, to little bump, wrong scale...  all help it look 3D.

Watch the tutorials over at GeekatPlay, show us what you are rendering, and keep asking how to make it better.  The best way to learn is to keep on Vueing!

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