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!
PLEASE NOTE that not all settings are effective on a per-user basis. For example, many settings only prior to the user's session being started.
Examples of settings which will not affect any behviour on a per-user basis include listen_address, banner_file, max_per_ip, max_clients, xferlog_file, etc.
default=none