Please note SFTP is only availaible to cpanel users and not to virtual ftp users for secure connection you can use FTP over SSL/TLS
Please enable TLS Encryption Support from WHM
Main >> Service Configuration >> FTP Server Configuration
TLS Encryption Support >> optional
On the FTP client use server type as
Host: domain or Ip
Port : 21
Server Type : FTPES -FTP over explicit TLS/SSL
For more information on this refer
http://forums.cpanel.net/f5/sftp-instructions-inside-cpanel-121025.html
http://docs.cpanel.net/twiki/bin/view/11_24/WHMDocs/FtpConfig
http://docs.cpanel.net/twiki/bin/view/AllDocumentation/CpanelDocs/FTPAccounts#Configure_an_FTP_Client
