Target:

• heach ftp user ca access a specified directory (chroot)
• each ftp user can access a subtree eventually overlapping another user subtree
• use virtual users - ftp users won't have access to the hosting machine and would be simply defined in a file (eventually allowing a simpler automation)

I use docker here to demonstrate the proposed setup; you can deploy the docker image if you wish or just follow the steps and reproduce it elsewhere.

What's in it:

• docker-compose.yml - following Yannick Pereira-Reis's suggestions compose is my preferred build tool for Docker
• etc/init.d is a modified copy of the init script in order to avoid the "vsftpd failed - probably invalid config." message as suggesterd here
• /etc/pam.d/vsftpd - pam configuration to instruct ftp daemon to look for users in /etc/vsftpd/ftpd.passwd (virtual users)
• /etc/vsftpd_user_conf contains an ad-hoc configuration file per each ftp virtual user
• /etc/vsftp.conf basic vsftpd configuration to enable virtual users and per-user-configuration
• Dockerfile is the magic glue for all this stuff

See

• how to setup virtual users for vsftpd with access to a specific sub directory
• Admin via web