I don't have the list of all the machine names, so I'd prefer a script that would discover all the machines on the network and do the update.
I realize the question is about modifying the host file via vbscript.
If you truly want EVERY user/machine on the network to see this change, perhaps this would be easier?
You can check the file properties to see if it is read only and you can check the file and etc folder properties security tab to see what permissions you have.
Select 'all files' and then choose the 'hosts' file. entirely possible that your user account does not have the necessary permissions to write to the directory or it is also possible that you have something like Spy Bot running that you have configured to "secure" the hosts file which it does by making it read only.
Open explorer and click on windows and then system32 and drivers and etc there is host file open it with notepad.
Where are you striking trouble you may need to if you are on XP open explorer and go to view folder options and tick/untick the option re showing hidden files/folders.