Well you really can't symlink below your /
When you create a default root it's setting the new environments root (much like a normal chroot). So now there is nothing below / which to you is /var/ftp but to anyone who ftp'd in is just /
You have several options though. You can either re-arrange mount points to be able to symlink from within the chrooted environment better. Maybe mount /var/ftp as one partition, and then /var/ftp/files as another and so on. This would require more work, but would be your best way of doing IMO.
Your other option is changing your chroot to something lower, something more along the lines of your actual / The huge problem with that is you've really got to watch for permissions then. You will want to set things up really secure, only allowing execution bit's to be set on the folders in which you want anyone to have acess to.