To be honest, if you’re doing stop motion, I think you’d be much better off importing your your files as image sequences. I assume your image files are numbered in some way? If so, go to File > Import > Media. In the Import dialog, select the very first image in the sequence, check the box labeled “Open still image sequence,” and enter a range if it’s different from the default. (By default, Vegas identifies what it thinks is the last image in the sequence. Then hit Open, and Vegas will insert all of the images as a single event. I believe it uses each image as a single frame, but of course you can time stretch the event(s) if you want them to remain on screen longer or maybe just change the frame rate for your project.
Rob