Serv-U FTP Server is described as 'Delivers quick, easy, and reliable file transfer for your organization. You can exchange sensitive files with your trading partners using secure FTPS (FTP over SSL/TLS) over both IPv4 and IPv6 networks' and is a FTP Server in the file sharing category. There are more than 10 alternatives to Serv-U FTP Server for a variety of platforms, including Windows, Linux, Mac, BSD and Self-Hosted apps. The best Serv-U FTP Server alternative is copyparty, which is both free and Open Source. Other great apps like Serv-U FTP Server are FileZilla Server, Microsoft IIS, SFTPGo and CrushFTP.
Serv-U runs on Windows; vsftpd is a linux based product. It could not be used as an alternative.