Here are my 2 cents:
As already explained, the dependencies are not produced by pacman, when using --as-deps just this attribute is set, but it does not form any link to another package, you cannot produce a dependency which is not specified by the other packages. 
vim-spell-en is not a vim dependency but the opposite
no reason to install with --as-deps, Explicitly installed is not a problem : when we remove vim we always remove vim-spell-** (as all vim-plugins)
you can run pactree vim
as writed, we flag package with --as-deps only for a clean auto remove : remove with app or set orphan
same with firefox and firefox-i18n-**