no, i do not want to disable line numbering. I want to avoid to copy them. I do not want to switch line numbering on off for every copy process. Absurd idea.
A better option would be to use a plugin that yanks text to the system clipboard.
You don't need a plugin for that. Just set the 'clipboard' option.
Note that you need Vim compiled with +clipboard for this to work (or +xterm_clipboard if you are using X11)