I'm interested in this as well since I just got a Vaio and am contemplating a fresh Windows 7 install myself.
Nearest I can tell, there are no direct hotkeys to disable the touchpad and the driver does not automatically disable it when a mouse is connected (unacceptable on a modern laptop, IMO)
The only way to disable it totally is through the Vaio Control Center software. It should be available for download from Sony's website. You just might have to get it from the page of a model that comes with Windows 7 (and, thus, has software designed for it)
My system is 64-bit and a different model than yours, but I don't really think that should matter for software too much. The file you're looking for is called Sony® Notebook Utilities
(Note that it also requires you to download the Sony® Shared Library
This seems like a lot of hassle to go through for something that should be a button-press away, but I can't think of any other way to do it. Let me know if the file works for you.