Scorcher24: That is not legit. Probably easier to download a Windows 10 image and entering the Vista license key. With a bit of luck, it still works. But a Windows 10 key for 3 bucks is NOT legit.
the_importer: WRONG! These keys are legit in the sense that they were bulked purchased from companies who did not need them after all. Now, of course, MS technically doesn't allow the resales of keys, but the fact of the matter is, they don't seem to give a shit since they're not doing anything about it. Worst case if they did something, you'd be out $3.
No, I am not wrong. They may work a short time, but they will stop working at some point and in the worst case, Microsoft is going to suspend your Microsoft Account, losing all content you may have. It is a big risk that shouldn't be taken. If you are looking for a discount, look for OEM licenses, however, they only work on one computer. It depends on the country if the standalone sale of those is legit or not. In Germany, it is.
Buying Volume Keys is not legit and is a risk - many of those keys will stop working within a month. It is bad advice that shouldn't be given. In many cases they are also stolen. Which makes you being in possession of stolen property.
I myself have a retail Windows Professional license and that is with me since Windows 7 came out. That one can be used on any number of computers.