I’m having a hard time picturing what it is you are trying to do. If you drop the image into a 1920×1080 project and go into Pan/Crop and select Match Output Aspect, Vegas will crop the images to match the video. You may have to adjust them up or down to get an exact match.
If you want them to behave as one piece of media you can make them into a separate project and then drop the project into your original project (like a nested comp in After Effects) and Vegas will treat the nested project like a single piece of media while still allowing you to edit the individual events separately in their own project.
~jr
http://www.johnrofrano.com
http://www.vasst.com