Thanks! I went with Mark van der Loo's answer, prebuilt the pdf vignette, saved it under vignettes/prebuilt.pdf and included it in vignettes/mypackage_pdf.Rnw:
\documentclass{article}
\usepackage{pdfpages}
%\VignetteIndexEntry{Using my package pdf version}
\begin{document}
\SweaveOpts{concordance=TRUE}
\includepdf[pages=-, fitpaper=true]{prebuilt.pdf}
\end{document}
I'll just have to remember to repeat the build and copy steps in the future 