It dosnt support it, it only supports specific listening ip addresses.
From the FAQ:And the /usr/share/doc/vsftpd/EXAMPLE/VIRTUAL_HOSTS/README only mentiones IP based virtual hosts.Q) Help! Does vsftpd do virtual hosting setups?
A1) Yes. If you integrate vsftpd with xinetd, you can use xinetd to bind to
several different IP addresses. For each IP address, get xinetd to launch
vsftpd with a different config file. This way, you can get different behaviour
per virtual address.
A2) Alternatively, run as many copies as vsftpd as necessary, in standalone
mode. Use "listen_address=x.x.x.x" to set the virtual IP.