Vegas Pro 12 allows you to pair all your images as stereoscopic 3D subclips (place them on two video tracks, select those tracks and choose “Create 3D subclip” command from context menu).
Set Playback Rate to 0.4 to slow down 60fps footage to get perfectly smooth 40% slow-motion in a 24fps project. Finally, don’t forget to increase the length of your video event.