I know that there is a software called vLite that can help me do this, but it requires me to have Vista installed in order to run it.
How can I customize an ISO image file of Windows Vista from a Windows 8.1 operating system?
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If you haven't yet, try installing and running vLite in Compatibility Mode by chosing Windows Vista. If the does not allow you to run it or it does not work properly, then I would suggest getting VirtualBox and installing Virtual Machine running Windows Vista. You can install and run vLite there and customize the ISO.