1. Download and install the trial version of UltraIso.
2. Plug in your USB key.
3. Right click on the UltraIso shortcut, and click on Run as administrator.
4. Click on File (menu bar) and Open. (See screenshot below)
5. Navigate to your Vista ISO file and select it and click on the Open button. (See screenshot below)
6. In the bottom Local pane, select your USB flash key drive. (See screenshot below)
7. Click on Bootable (menu bar) and Write Disk Image. (See screenshot above)
8. Make sure the locations are correct, and check the Verify box and click on the Write button. (See screenshot below)
9. Click on Yes to confirm writing to the USB drive.
10. When it's done, click on the Close button. (See screenshot below)
11. You now have a Vista installation USB flash key drive.
Source : http://www.vistax64.com/tutorials/181538-usb-bootable-vista-installation-flash-thumb-drive.html