try if this will work for your needs:
function! SetTmuxBuffer() range
execute "!tmux set-buffer '" .join(getline(a:firstline, a:lastline),"\015")."'"
endfunction
command! -range ToTmux <line1>, <line2> call SetTmuxBuffer()
vimrc vim . , , line1-line8 tmux:
:1,8ToTmux
,
:'<,'>ToTmux
tmux, vim.