First, understand the Vista to Win 7 upgrade path.......
Secondly, you need to make sure if your Vista is capable to run Windows 7.
To do that, run the Windows 7 Upgrade Advisor:
You shouldn't have problem with this stage. But, it is better to run it and be sure. You don't want to go out and buy the Win 7 and then find out your machine can't run it.
Excerpt : It scans your PC for potential issues
with your hardware, devices, and installed programs, and recommends what to do before you upgrade.
Thirdly, you must have a valid Win 7 product key
to do the upgrade. Your Vista product key is no good for this purpose. You will have to pay for that Win 7 product key.
Lastly, please read this detail tutorial regarding Upgrade Install from Vista to Win 7. Also note that Clean Install is also available. Link to Clean Install tutorial is at the Information section.