How can I modify vsftpd.conf in macOS?
You should have gotten the following output while installing vsftpd:

To use chroot, vsftpd requires root privileges, so you will need to run
  sudo vsftpd.
You should be certain that you trust any software you grant root privileges.

The vsftpd.conf file must be owned by root or vsftpd will refuse to start:
  sudo chown root /usr/local/etc/vsftpd.conf

To have launchd start vsftpd now and restart at startup:
  sudo brew services start vsftpd
Or, if you don't want/need a background service you can just run:
  sudo vsftpd


So your vsftpd.conf file is in /usr/local/etc.