Create your own bootable Vista iso image
For a complete installation, you'll need three files: an *.exe file plus the two *.wim files. If you want to upgrade an existing Windows system, you can start the Vista installation by executing the *.exe and following the instructions.
You might also want to have a bootable Vista DVD iso image. Microsoft doesn't provide these as free downloads, but actually it's quite easy to create an iso image from the setup files. Just download the setup files, start the *.exe, and wait for the Windows Vista system files to unpack to a new folder. Then cancel the Vista setup process. Let's assume the setup files have been unpacked to C:\Vista\. If they're in a different folder, just replace it in the following steps.
To create an iso image, you'll also need to download and install the Windows Vista SP1 WAIK. Download Automated Installation Kit (AIK) for Windows Vista SP1 and Windows Server 2008 from Official Microsoft Download Center
If you're creating a 64 bit iso image (and only if doing so), you'll need to copy the file C:\Program Files\Windows AIK\Tools\PETools\amd64\boot\etfsboot.com to the folder C:\Vista\boot\.
You can then trigger the iso creation by opening Windows PE Tools, which will give you a command prompt. There you enter the following line:
oscdimg –bC:\Vista\boot\etfsboot.com -h -lVISTA_EN_DVD -m -u2
C:\Vista\ C:\windows_vista_with_service_pack_1_dvd.iso