vertically center subfloat figure

Question

How can I vertically center the taller figure on the right? I tried reading subfig package and there mentioned \savebox but I'm not sure if that can provide a solution. Please help. Thanks.

\documentclass{report}
\usepackage[utf8]{inputenc}
\usepackage{graphicx}
\usepackage{array,booktabs}
\usepackage[version=4]{mhchem}
\usepackage[margin=0pt]{subfig}
\usepackage{caption}

\begin{document}
\begin{figure}
\begin{minipage}[c][11cm][t]{.5\textwidth}
\vspace*{\fill}
\centering
\subfloat[a]{\includegraphics[width=5cm]{./graphics/plots/ST034-1-4SG.pdf}
}
\par\vfill
\subfloat[b]{\includegraphics[width=5cm]{./graphics/plots/ST034all.pdf}
}
\end{minipage}
\begin{minipage}[c][11cm][t]{.5\textwidth}
\vspace*{\fill}
\centering
\subfloat[c]{\includegraphics[width=5cm]{./graphics/plots/ST034-SEC.pdf}
}
\end{minipage}%
\end{figure}
\end{document}

Answer 1

Your two minipages with little modifications can do the job (I additionally suppressed some spurious blank spaces)

\documentclass{report}
\usepackage[utf8]{inputenc}
\usepackage[demo]{graphicx}
\usepackage{array,booktabs}
\usepackage[version=4]{mhchem}
\usepackage[margin=0pt]{subfig}
\usepackage{caption}

\begin{document}
\begin{figure}
\begin{minipage}[c]{.5\textwidth}
\centering
\subfloat[a]{\includegraphics[width=5cm]{./graphics/plots/ST034-1-4SG.pdf}}\par
\subfloat[b]{\includegraphics[width=5cm]{./graphics/plots/ST034all.pdf}}
\end{minipage}%
\begin{minipage}[c]{.5\textwidth}
\centering
\subfloat[c]{\includegraphics[width=5cm]{./graphics/plots/ST034-SEC.pdf}}
\end{minipage}
\end{figure}

\end{document}

The demo option for graphicx simply replaces actual figures with black rectangles; do not use that option in your actual document.

Answer 2

You can better control the horizontal spacing by using tabular, exploiting the fact that by default it does vertical centering of the material with respect to the current line of text; instead of \qquad you can use any \hspace you want.

For adding space between the two images on the left, you can use

\addlinespace[2cm]

or whatever length, between the two rows (after \\, to be explicit).

\documentclass{report}
\usepackage[utf8]{inputenc}
\usepackage[demo]{graphicx}
\usepackage{array,booktabs}
\usepackage[version=4]{mhchem}
\usepackage[margin=0pt]{subfig}
\usepackage{caption}

\begin{document}

\begin{figure}

\centering

\begin{tabular}{@{}c@{}}
  \subfloat[a]{\includegraphics[width=5cm]{./graphics/plots/ST034-1-4SG.pdf}}\\
  \subfloat[b]{\includegraphics[width=5cm]{./graphics/plots/ST034all.pdf}}
\end{tabular}\qquad
\begin{tabular}{@{}c@{}}
\subfloat[c]{\includegraphics[width=5cm]{./graphics/plots/ST034-SEC.pdf}}
\end{tabular}

\end{figure}

\end{document}

The demo option is just for getting an output without your images.