listings with background color and mathescape broken

I have a listings environment in my document with backgroundcolor=\color{grey} to get a nice grey background and mathescape=true for using math symbols. However when using the \vdots command inside a code listing I get a break in my listing, as follows:

\documentclass{book}
\usepackage{color}
\definecolor{grey}{rgb}{0.9,0.9,0.9}
\usepackage{listings}
\lstset{%
language=Lisp,
basicstyle=\small,
backgroundcolor=\color{grey},
mathescape=true}

\begin{document}
\begin{lstlisting}
(let ((<$var_1$> <$exp_1$>)
(<$var_2$> <$exp_2$>)
$\vdots$
(<$var_n$> <$exp_n$>))
<$body$>))
\end{lstlisting}
\end{document}


Resulting output:

So far it's only \vdots which is doing this (and it happens multiple times so I know it's not a fluke). Is there a fix for this, or maybe would minted work better at this sort of thing? (Although I'd rather not switch if I don't have to.)

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Nice question! You could improve the question by providing compilable code with \documentclass and everything. –  Hendrik Vogt Feb 13 '11 at 6:59
@Hendrik - Thanks! Ask and ye shall receive. –  Andrew Arnold Feb 13 '11 at 7:09
And suddenly I don't get Undefined color grey' anymore :-) –  Hendrik Vogt Feb 13 '11 at 7:34

The problem seems to be that listings expects a constant line height, and that \vdots just takes up too much vertical space. One fix is to redefine \vdots so that it takes up less vertical space:

\renewcommand\vdots{%
\vbox{\baselineskip3pt\lineskiplimit0pt\kern1pt\hbox{.}\hbox{.}\hbox{.}\kern-1pt}}


-
Looks very nice! Thanks for your help. –  Andrew Arnold Feb 13 '11 at 13:36
\documentclass{book}
\usepackage{color}
\definecolor{grey}{gray}{0.9}
\usepackage{listings}

\lstset{%
language=Lisp,
basicstyle=\small,
backgroundcolor=\color{grey},
mathescape=true}

\begin{document}
\begin{lstlisting}
(let ((<$var_1$> <$exp_1$>)
(<$var_2$> <$exp_2$>)
$\makebox(0,7.5){\vdots}$
(<$var_n$> <$exp_n$>))
<$body$>))
\end{lstlisting}
\end{document}

-
This solution eats a bit of the lowest of the three dots. My opinion: Here, a bit of explanation what \makebox does would be much more valuable than the full code. –  Hendrik Vogt Feb 13 '11 at 9:35
This doesn't quite work in my document since it shoves the dots to the far left of the page, eating up any indentation. On the other hand, I learned something new about \definecolor`! –  Andrew Arnold Feb 13 '11 at 13:47