Veja a diretiva user_config_dir
.
This powerful option allows the override of any config option specified in the manual page, on a per-user basis. Usage is simple, and is best illustrated with an example. If you set user_config_dir to be /etc/vsftpd_user_conf and then log on as the user "chris", then vsftpd will apply the settings in the file /etc/vsftpd_user_conf/chris for the duration of the session. The format of this file is as detailed in this manual page
por exemplo.
user_config_dir=/etc/vsftpd/user_config
Você deve ser capaz de criar um arquivo para o usuário externo e especificar
write_enable=NO
para impedir a capacidade de escrever.