What is expected from the server by WinSCP to detect it? Or perhaps WinSCP detection mechanism from FTP is not good enough solution and WinSCP should be changed?
WinSCP looks for
FEATresponse. What the latest version of vsftpd does.
If it does not work for your, please attach a full session log file showing the problem (using the latest version of WinSCP).
To generate the session log file, enable logging. Submit the log with your post as an attachment. Note that passwords and passphrases not stored in the log. You may want to remove other data you consider sensitive though, such as host names, IP addresses, account names or file names (unless they are relevant to the problem). If you do not want to post the log publicly, you can mark the attachment as private.