This simple weechat plugin allows you to compose messages in your $EDITOR.

/edit
# Type some stuff
# Save and quit

If you'd like to customize the editor you use outside of the $EDITOR environment variable, you can set it in weechat.

/set plugins.var.python.edit.editor "vim -f"

In case you want to run editor externally without blocking weechat (since blocking weechat can break things), you can configure the plugin like this:

/set plugins.var.python.edit.editor "gvim -f"
/set plugins.var.python.edit.run_externally "true"

You can of course use any editor you want, you can even spawn a terminal and use terminal vim if you prefer.

Copy the script to ~/.weechat/python/autoload

mkdir -p ~/.weechat/python/autoload
wget https://raw.githubusercontent.com/keith/edit-weechat/master/edit.py ~/.weechat/python/autoload