Vista upgrade genuinity question

It will be, but installing that way is against the EULA.

Partially correct -- It is only against the EULA if you do not have a qualifying product that you are retiring in the process --- the actual clean install of the OS is perfectly within the EULA if you do have such a product - there is no requirement to have that copy physically installed on the system prior to upgrading you just need the product key from that product as a record that you were indeed qualified to use the upgrade version.

Also @ OP --

So I clean install windows vista, then upgrade from that to vista so that it is believed that I am upgrading from a previous windwos product? Or can I just clean install, then run slui.exe 4 and call and give my windows xp product key?

You can clean install from the upgrade media (using any of the numerous guides that explain how to do a clean install from upgrade media -- Here is a good article that describes the process ) - using the upgrade key or using no key at all (Win 7 installs give you IIRC 30 or 60 days to activate even if you install without a key) then activate the install afterwards -- most of the time the install will validate properly using either the online process or the automated phone activation but if those fail then you just call and speak to a rep and give them the XP or Vista Product key along with the WIN 7 Upgrade key and they will give you a response code to enter to activate the installation ( usually the online process or the automated phone activation will work but on the odd occasion that it does not as long as you have the Vista key you are upgrading and the upgrade version Key the rep will give you an activation code once you talk to them)