Very secure, but hard to configure for virtual users and chroot.
Here i have crack it down one day for my client.

| /etc/vsftpd/vsftpd.conf

anonymous_enable=NO
local_enable=YES
write_enable=NO
anon_upload_enable=NO
anon_mkdir_write_enable=NO
anon_other_write_enable=NO
pam_service_name=ftp
chroot_local_user=YES
guest_enable=NO
guest_username=www
listen=YES
pasv_min_port=30000
pasv_max_port=30999
user_config_dir=/etc/vsftpd/vsftpd_user_conf
userlist_enable=YES
userlist_file=/etc/vsftpd/denied_users

• create directory “/etc/vsftpd/vsftpd_user_conf”
• create file “/etc/vsftpd/vsftpd_user_conf/www”

| /etc/vsftpd/vsftpd_user_conf/www

write_enable=YES
dirlist_enable=YES
download_enable=YES
local_root=/var/www/html

• create pam auth rule for db4 based database

| /etc/pam.d/ftp

auth    required /lib/security/pam_userdb.so db=/etc/vsftpd/vsftpd_login
account required /lib/security/pam_userdb.so db=/etc/vsftpd/vsftpd_login

• crate text file with username / password. users.txt

| /etc/vsftpd/users.txt

www
wwwpassword
username2
password2

• generate database based on users.txt file

db_load -T -t hash -f logins.txt /etc/vsftpd/vsftpd_login.db

• generate virtual user restriction for second user

| /etc/vsftpd/vsftpd_user_conf/username2

local_root=/var/www/html/xyz.com
dirlist_enable=YES
download_enable=YES
write_enable=YES

DONE