How do I make sure sftp server is VSFTPD?
Solution 1:
For completeness sake, as the issue was mostly linguistic and already resolved from my comment.
By configuring an SSL certificate in VSFTP you're setting up an SSL secured FTP server. That protocol is FTPS.
That protocol is often confused with the incompatible SSH File Transfer Protocol; SFTP. You obviously can't test the VSFTP configuration with an sftp client.
As a footnote, a good multi-purpose tool for transferring data that supports both protocols and more is curl.