Using progressive 1080p images in Sony Vegas Pro does not always give you the high resolution results that you may have expected. This is due to the AVI defaults within Vegas that assumes all HD sources are Upper Field first interlaced. Vegas then deinterlaces the image to be compatible with the progressive project -- resulting in the loss of your vertical resolution.
The simple solution is to right click on the imported clip and bring up the properties panel, changing the "Field Order" to progressive. This works fine, yet you have to do for every clip manually. To make these change occur for every new imported clip, bring the first clip into the project, setting the properties to progressive and then click on the "Save Settings" icon before closing the properties panel. Each new imported clip that shares the properties of the first (resolution and frame rate) will automatically receive the corrected settings. Your full resolution is now preserved.