1. Edit /etc/vsftpd.conf as follow.
/etc/vsftpd.conf

chroot_local_user=NO
chroot_list_enable=YES
chroot_list_file=/etc/vsftpd.chroot_list

2. Create the /etc/vsftpd.chroot_list and enter the usernames which you want to jail.
/etc/vsftpd.chroot_list

userA
userB
userC
...

3. By default, the chroot folder has to be non-writable. The simplest way is to change the user home folder permission but this is not a perfect solution. If you are looking for a better solution. please refer to this.

chmod a-w /home/<username>

4. Restart the vsftpd.

/etc/init.d/vsftpd restart

5. Connect to the server using any FTP client, the jailed FTP account should work.

Done =)

Reference:

• How to create an FTP account for one folder?
• Ubuntu 9.10: Chroot Jail for FTP
• Fixing 500 OOPS: vsftpd: refusing to run with writable root inside chroot ()