Just an idea or thought on it. You never said what the file size was or did you? My idea is that is, there not a file size limitation on transfer to and from the server? I thought there was? Check you vsftpd.conf out or what ever ftp server your running for the size limitation. Maybe some help or maybe not?
The problem is with SFTP, so I'm afraid that vsftpd.conf isn't the culprit here.