In a VBScript custom action, WScript object cannot be referenced. Hence, the script should be as followed. In addition, I will leave out
On Error Resume Next during testing, and include only if errors during script execution aren't critical to the installation process.
Set objShell = CreateObject ("WSCript.shell")
Automation Interface Reference will be a good starting point for VBScript custom action. Of all, Session object will used extenisvely.