I only have chroot_local_user enabled on my server, so I don't really know if the following will work or how to implement it exactly, but this is from the docs
If enabled, along with chroot_local_user , then a chroot() jail location may be specified on a per-user basis. Each user's jail is derived from their home directory string in /etc/passwd. The occurrence of /./ in the home directory string denotes that the jail is at that particular location in the path.
Check out man vsftpd.conf for more stuff on chroot'ing