I hope someone corrects me on this but AFAIK it is very difficult to work with vsftpd (the standard ftp server in Zentyal), as it chroots users to their home folders.
I found that using Samba shares works much better, as Zentyal provides a finer grade of control for specific folders and users and groups.
To have remote access, I opted for a VPN, as it will provide me with an encrypted connection and this way I don't have to leave Samba ports open on the router for everyone to see.