I think you should replace
\citep[vgl.~bspw.][S.~380]{Bortz2006}\citep[S.~40]{Fayyad1996}\citep[S.~11]{Maimon2010}
with
\citetext{vgl. bspw. \citealp[S.~380]{Bortz2006}; \citealp[S.~40]{Fayyad1996};
\citealp[S.~11]{Maimon2010}}
Observe the use of \citetext
and three \citealp
instructions.
A full MWE (minimum working example):

\documentclass{article}
\usepackage[numbers]{natbib}
%\bibliographystyle{lni}
\frenchspacing
\begin{document}
\citep[vgl.~bspw.][S.~380]{Bortz2006}\citep[S.~40]{Fayyad1996}\citep[S.~11]{Maimon2010}
\medskip
\citetext{vgl. bspw. \citealp[S.~380]{Bortz2006}; \citealp[S.~40]{Fayyad1996}; \citealp[S.~11]{Maimon2010}}
\begin{thebibliography}{99} % just for this example
\bibitem[BD06]{Bortz2006} stuff
\bibitem[FPSS96]{Fayyad1996} stuff
\bibitem[MR10]{Maimon2010} stuff
\end{thebibliography}
\end{document}
natbib
defaults. E.g., do you set thesquare
option when loading the package? – Mico Aug 15 '16 at 9:41