MWE:
\documentclass{article}
\usepackage{geometry}
\newcommand{\GeomOpts}{a4paper,vmargin={3cm,3cm}}
\geometry{\GeomOpts}
\begin{document}
\end{document}
Because of the braces in \GeomOpts its expansion fails when inserted as parameter to \geometry. I did read [1] and [2] but I didn't find a way to adapt this to my simpler \geometry case.
I did try with \expandafter before \geometry and before its braces. Also I did try with xparse's commands intented to define macros/command without success.
[1] Keyval Error with \includegraphics: options undefined
[2] Strip one level of braces for using keyval-value within nested keyval
\begingroup\edef\x{\endgroup\noexpand\geometry{\GeomOpts}}\x
\x
local and to use the power\edef
.