How do I add a new user to vsftpd and let root access the FTP server?

Follow the manual for adding new users. That's why they are written.

You won't allow root to access the FTP server! This is the highest security risk you can expose your host to. FTP transmits the password in clear text over the wire. Everybody having access to the network can easily sniff the password. Having the password you give everybody full control over your server! In other words: Don't do that!!

SFTP is another protocol and has nothing to do with FTP beside the name. A protocol related to FTP is FTPS which is nothing else than FTP over SSL. SFTP is something like FTP alternative over SSH.

The method you have outlined will work for creating ftp users. You may need to change the Selinux policy to Allow ftp to read/write in the user home directories

sudo /usr/sbin/setsebool -P ftp_home_dir 1

Regarding letting root log in over ftp or ftps - DON'T just don't. The fewer places root can access your system from the safer it is.