There isn't any legal way to download the Windows Vista software because Microsoft never sold it online nor were there any legitimate vendors. Windows 7 was the first online distribution of Windows by Microsoft.
You can find a boxed copy of Windows Vista from sellers on Amazon.com, but supply is unstable. Some Torrent sites distribute ISO format copies of Windows Vista, but they might have malware or other issues. GetIntoPC.com has both copies of 32-bit and 64-bit of Windows Vista Home Premium, in ISO format. However, they might not have explicit permission from Microsoft to distribute those copies, and you still need a product key for installation.
You can try and contact Microsoft, and ask them for a replacement disc, or visit your local computer store and look for one there. If your computer came pre-installed with windows vista, contact the manufacturer, and ask for a Restore Disc or a replacement Vista DVD but make sure you have your Vista product key and date of purchase ready, before contacting them.
If you have Windows Vista already installed on your computer, you can get free updates or download the Windows Vista Service Pack 1 from Microsoft.com, or enable the Windows Update feature for automatic updates.