It's always a good practice to back up the ESXi host configs before any updates/patches are rolled out to it. However, every single time I do it I always end up with some syntax/wrong parameter error.
For reference, running the following command will backup the ESXi host configs onto the local desktop. From my experience, this configuration file can only be used to restore the ESXi configs onto an ESXi host of the same version.
"C:\Program Files (x86)\VMware\VMware vSphere CLI\Perl\bin\Perl.exe" "C:\Program Files (x86)\VMware\VMware vSphere CLI\bin\vicfg-cfgbackup.pl" --server esxi-server --username root -s c:\esxi-server.bak