Since devtools 2.0.0 update, there are some changes in behaviour.
The function devtools::install_github is now using remotes::install_github. This function has no build_vignettes = TRUE argument and by default vignette is not built.
However, there is a build_opts argument controlling the build process. By default, you'll see in help page that this argument contains by defaut --no-build-vignettes. You can remove this options to get the vignettes.
So the workaround with the new version is
remotes::install_github(..., build = TRUE, build_opts = c("--no-resave-data", "--no-manual")
# or with devtools >= 2.0.0
devtools::install_github(..., build = TRUE, build_opts = c("--no-resave-data", "--no-manual")
currently, there are some discussions on this new behaviour for devtools