By default, sony vegas resizes all the photos and videos into the frame. If, let's say, your footage is in 720/1080p, you can go in your video event's 'Pan/Crop' and insert 720x480 into the event's width and height respectively. The frame will automatically position itself to the center of the frame. Now you can go ahead and move the frame within the HD footage.
If your HD footage is the same aspect ratio (16:9) you can make it easier by locking the aspect ratio of the frame (5th option on the left side of the 'Pan/Crop' effect window).
Let me know if you encounter any more problems.