What I take from all I have learned now is that Vue will not mimic the function of a camera correctly with regard to depth of field. There is always a line marking the back of the sharp area. Thus, as my background is quite far back... I'll have to use other methods.

Thank you all for helping me sort this out. At least I know now that it's not just me being too stupid to handle the thing ;-)

(attached image: Blur 0% and 4% for comparison and documentation)