\ifthenelse within \caption, \listoftables does not work

Question

EXAMPLE 1 below works fine but EXAMPLE 2 produces "undefined control sequence". The only difference is the \vlabel command within brackets in the \caption command. What is the problem?

EXAMPLE 1:

\documentclass{article}
\usepackage{pgffor}
\usepackage{ifthen}

\newcommand{\vlabel}[1]
{%
\ifthenelse{\equal{#1}{numbersa}}{Numbers A}{}%
}

\begin{document}

\listoftables

\begin{table}
\caption[Numbers A]{\vlabel{numbersa}}
\end{table}

\end{document}

EXAMPLE 2:

\documentclass{article}
\usepackage{pgffor}
\usepackage{ifthen}

\newcommand{\vlabel}[1]
{%
\ifthenelse{\equal{#1}{numbersa}}{Numbers A}{}%
}

\begin{document}

\listoftables

\begin{table}
\caption[\vlabel{numbersa}]{\vlabel{numbersa}}
\end{table}

\end{document}

Answer 1

The expansion of ifthen macros causes problems in the auxiliary files. You either need to \protect them

...
\caption[\protect\vlabel{numbersa}]{\vlabel{numbersa}}
...

or use an alternative:

\documentclass{article}
\newcommand{\vlabel}[1]{%
  \ifnum\pdfstrcmp{#1}{numbersa}=0
    Numbers A%
  \fi
}

\begin{document}

\listoftables% LoF

\begin{table} \caption[Numbers A]{\vlabel{numbersa}} \end{table}

\begin{table} \caption[\vlabel{numbersa}]{\vlabel{numbersa}} \end{table}

\end{document}

\pdfstrcmp (requires e-TeX) is expandable, making sure that only Numbers A makes its way into the LoF.