Thank you very much, it's working now!

I didn't even think about setting cullmode to 'cw' for the outline, although I tried setting it to 'none' to get less flaws in the outline.

Do you know how to fix the flaws in the outline? Or wouldn't it be possible to get a 'flawless' outline using this approach?

Oh, and to get back to my first question - is there another way of scaling an object from the vertex shader? Without using the vertex normals.