You can install DISM as part of the Windows ADK (Assessment and Deployment Kit). See the system requirements and installation instructions…Vista is supported
http://www.microsoft.com/en-us/download/details.aspx?id=30652
If this fails, your options are limited.
Unfortunately, vendor recovery disks will re-install Windows and destroy all your personal files, folders, and settings. In other words, it restores it back to out-of-the-box condition.
If you can get your hands on a correct version and bit-ness (32 bit) of a Windows Vista disk, you can do a non-destructive re-install of Windows without losing your personal files, folders, and settings. Do a search for Fred Langa’s article on how to do this. You will need your license key.
Your last resort would be to copy all your personal files and folders to removable media, and then use the vendor recovery disk to do a complete re-install of Windows Vista. All of your programs would need to be re-installed, and you would be doing Windows Updates for a while, but a least your personal files would be saved.
This type of situation is an example of why you need disk imaging software, and a removable hard drive to back up your computer. Some 1 TB removable drives are less than $100 US now. Disk imaging software like Terabyte Unlimited’s Image for Windows (my choice) is $39 US.
I do a full disk image once a week, and backup individual files that have changed on a daily basis, or sometimes several times a day. So at most, I am one day behind on anything I’m working on.
It takes me 20 minutes to restore a full disk image, over a network connection.
Best of luck.