# Why does the \vspace command not work with PSfrag?

When loading in eps graphs, I wanted to shift the positions of the axis labels, and I found I could achieve that with PSfrag (something like \psfrag{label}{\hspace{1em}label}). I've found that this works with \hspace but not \vspace. I now know that instead of \vspace, I should use \raisebox{1em}{label}, thanks to this lovely answer. Does anyone know why?

A psfrag replacement is essentially set in an \mbox so a horizontal mode contsruct, as always in such a construct vertical mode material doesn't work, although you could nest a \parbox to get into vertical mode and add a vertical material. or more simply you could use \raisebox{10pt}{...} rather than using \vspace