I experienced a similar issue recently. When I was using Internet Explorer 11 to access an FTP URL, it would not prompt for a username and password. If I used Firefox it worked fine. I figured out the issue in my case. Since I set the vsftpd setting userlist_deny=NO, the file user_list (/etc/vsftpd/user_list) became a file containing a list of users that are allowed to connect to the server. An entry for user "anonymous" had to exist in there for some browsers to work properly, such as Internet Explorer 11. I had to do that even though I don't allow anonymous access by setting anonymous_enable=NO.

The reason is the following. When you use a web browser to connect to an FTP URL, it will first try to automatically connect as user anonymous. If that user is not explicitly authorized to connect to the vsftpd, which is needed when using userlist_deny=NO, the browser will get a permission denied response. Some browsers don't handle this well, such as Internet Explorer 11. That browser stopped trying to connect immediately so it didn't ask me for a username and password. Once I added user anonymous to the authorized list, instead of the browser receiving a permission denied response it received a response asking for the password of user anonymous. Once the browser couldn't log on automatically as user anonymous it prompted me for a username and password.