The automatic bookmarks
'> get updated only after the Visual mode is left. Hence
<Cmd> is not useful for your example, as you effectively apply the previous selection range, not the current one.
You really should use "normal colon"-mapping here:
xnoremap <Leader>c :w !xsel<CR>
Note (1) use
xnoremap instead of
vnoremap so we don't mess with "Select" modes; (2) don't add "Visual" marks, as they are put in automatically; (3) do
:w !xsel to have stdin redirected only, as we don't need xsel's output.
Also note that Neovim is normally able to find xsel/xclip and to set up "plus" and "star" registers without any additional scripting (see