DVOM,it does matters in which order they were installed.I don't think it matters in what order they were installed. I would use a bootable partition editor (like a live linux CD) to "hide" the Vista partition. (you could just delete it but "hiding" it allows you to recover it if necessary)
Then I'd boot to the Windows 7 DVD and run start-up repair.
Then, if everything runs/boots smoothly, go ahead and delete the Vista installation.
Edit: Ooops, forgot they're on two different drives, so (as has been said) just unhooking the Vista drive and then doing a start-up repair will work. However, my method will work with separate drives or separate partitions.
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