1. Before starting reset root password process make sure to take the snapshot of VMware vCenter server appliance virtual machine.
2. Connect on ESXi host where vCenter running and reset vCenter then press 'e' to enter in GRUB menu, now add to end line where is $systemd_cmdline:
3. Press CTRL+X or F10 to boot, when You got bash can reset root password with:
mount -o remount,rw / passwd umount / reboot -f
Once root password is reset successfully, you can use ssh to connect vCenter server, if connection is successful, you can delete snapshot from VM.