I think most people refer to it as shaded vector art. It's popular in icon design. Google "shaded vector icons," and you'll get a lot of examples.
More artsy people would describe it as simple Geometric Vector art. A geometric style is characterized by the use of simple shapes (triangles, squares, circles, abstract, etc.) to form patterns or more complexed shapes.
Here's a few more examples:
https://www.behance.net/gallery/1423547 ... e-Portrait