The short answer to your question is yes. Put in your Vista upgrade first and start the install and at some stage it will ask you to put in your XP disc to prove that you are entitled to an upgrade to Vista. You do not need to install XP first before installing the Vista upgrade.
I have upgraded several machines with different Vista Upgrade disks and original XP disks and each time I have been asked to put in the XP disk as a verification check. It only takes a few seconds.
Yes it is well documented that you can use a Vista Upgrade disk without an old XP disc click here but this is a longer process as you basically install Vista by ignoring the request to enter the Product Key and then do it all over again effectively upgrading the upgrade. Just as clean but a longer process as you install Vista twice.
If you have an XP disc it is much quicker to just put this in when asked.
I can only speak from my experience. I purchased an upgrade version of Vista. I installed it on a new 500Gb hardrive. I booted up with the Vista disc in a CD drive and did the format. At some stage - I can't tell you exactly when - I was asked ot insert my XP disc. I clearly remember this as the XP disc itself is an upgrade disc bought many years ago to upgrade from Windows 98 and I wondered if it would work. It did and I removed it, put back the Vista disk as requested on the on screen instructions and continued with the Vista install. That was my experience and I based my reply to Nessie on that.
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