Let me start at the very beginning:
My project frame rate is 23.976 fps (standard film). When I create such a project in Adobe Premiere and then want to add a clip to the timeline, I can set the frame rate of the clip manually. Therefore, when my clip is 60 fps shot with a Canon DSLr (actually I believe and hope it's 59.94 fps), I can override its fps to 23.976 and then drop it to the timeline, thus getting the perfect slow-motion = as slow as possible without any frames missing in-between.
Now, I don't know how do I do this in Vegas Pro 11. All I can do is drop the clip to the time-line and then set its playback rate to 0.4x original speed (59.94 * 0.4 = 23.976). However, I don't know how safe this is. Is it safe to assume that the playback speed of the clip will be EXACTLY 23.976 fps and there won't be any frames missing in-between, thus creating the perfect slow-motion?