In your vsftpd.conf file, look for the directive anon_root
from the man 5 vsftpd.conf
This option represents a directory which vsftpd
will try to change into after an anonymous login.
Failure is silently ignored.
I would guess just add a line like this
and you should be good. It should probably go before the other anon lines (anon_world_readable_only, anonymous_enable, etc) but I'm not sure if it makes a difference.