You may be running into permissions issues. Since this is being performed on the local computer, it is done under the permissions of the logged on user. If the user does not have Administrator level permissions on the computer, they can not install the printer port or the drivers. Power Users will not have the required level of permissions.
You can perform this installation remotely using what you are already doing but by adding the correct switches to the VBScript.
cscript.exe prnport.vbs -a -r PortName -s ComputerName -h IPAddress -q QueueName
Refer to this
MS TechNet article for more information on PrnPort.vbs
By doing this remotely, you are performing this using the permissions of your account (which I assume is a Domain Administrator). It is much easier to use a Windows Server for your printers and install the printers as shared devices. The users will only need Power User permissions on their PC's to add the printers and the drivers are centrally managed at the print server for all users.
A good link for scripting is the
Microsoft Scripting Guys.