You can use the Win7 DVD from the second laptop to install Windows 7 on the first (Vista) one.
However, you cannot use the product key on the first laptop to activate Windows 7, because the key is for Windows Vista (assuming that's what the laptop originally came with). Also, you cannot use the Windows 7 key on the second laptop to activate Windows 7 on the first one, as OEM keys are tied to the computer (motherboard, to be exact) they were shipped with and cannot be used on any other system.
Bottom line: if you want to install and use Windows 7 on your Vista laptop, you'll need to buy a new Windows 7 product key. You can use the Windows 7 DVD you have to do the actual installation. Note that the key you buy will have to be the same edition that the DVD is for (ie. if the DVD is Home Premium, you'll have to buy a Home Premium product key).
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