there is an option for that, you just have to add
let g:tex_comment_nospell=1
in your .vimrc
.
About the internals, this variable is used in the file /usr/share/vim/vim73/syntax/tex.vim
in my installation.
[Edit for older versions]
Checking vim source tree in older versions, this variable appears from revision 1073, which is after vim 7.0 and 7.1. As stated by the diff, this modification introduces the lines
if !exists("g:tex_comment_nospell") || !g:tex_comment_nospell
syn cluster texCommentGroup contains=texTodo,@Spell
else
syn cluster texCommentGroup contains=texTodo,@NoSpell
endif
so maybe adding
:autocmd BufEnter *.tex syntax cluster texCommentGroup contains=texTodo,@NoSpell
in your .vimrc
would do the trick, at least it works on vim 7.3. If this does not work, you could try to get a more recent syntax file and put it into .vim/syntax/tex.vim