There doesn't seem to be any way to do this. Vegas 3 had a "stretch to fit frame" option that could be turned off. This feature doesn't seem to exist in Vegas 4 or 5.
If you go into Pan/Crop, under the Position attributes, set the Width to match your project width (e.g., 720) the image will be resized to whatever its original size was. (this basically un-zooms it)
But if you are working with lots of stills this is like painting a house with a toothbrush.
This is a "bug-like feature" and can't be turned off. There are ways to get around it in your workflow. One way to deal with it is to paste the images onto appropriately sized canvases before you import them. You an use 655x480 or any multiple of that. 720x528 is a nice multiple, for instance.