must specify the vignette engine explicitly

as documented in the R-exts manual: http://cran.r-project.org/doc/manuals/r-release/R-exts.html#Non_002dSweave-vignettes

currently CRAN has disabled building your vignettes perhaps due to this reason: http://cran.r-project.org/web/checks/check_results_ChemoSpec.html