I have had this come up before and the only way to resolve was to do the following:
1. Server needs to be in the same vlan as the BIG-IP (which you do not have)
2. Attach the ftp virtual server IP to the loopback interface on the ftp server
3. Configure FTP server to listen on the virtual server IP that is attached to its loopback interface
4. Configure the virtual server to NOT translate IP.
5. Server uses BIG-iP as the default gateway
Since FTPS is secured between the client and the server, there is not much BIG-IP can do as a device in the middle without visibility into the FTP session.