Vegas Movie Studio stopped recognizing iPhone .mov files after upgrade to iOS11
Apple made changes to their image and video formats in iOS 11. For photos, instead of JPG they now use High Efficiency Image File Format (HEIF). For videos, instead of H.264 they now use High Efficiency Video Codec (HEVC).
As a result, Vegas (Sony) Movie Studio no longer recognises .mov files created by iPhones and iPads running iOS11 (or imports them as audio-only clips). As a workaround you can tell iOS to record videos in the old H.264 .mov format. To do this go to Settings > Camera > Formats and choose Most Compatible.