I am a user of Sony Vegas Pro, version 13, and recently I have come across a problem. When I import an MP4 video and attempt to play it back, the video will show up in the preview and the audio will work just fine but it will display as only an audio-track instead of both the audio track AND video track. Therefor I am unable to add effects, lower opacity, or greenscreen the video which is rather inconvenient. This happened very suddenly whilst editing; I was not interfering with the program in any other way. Thank you for all future replies.
Maybe possibly drag the preview window up, (or use the adjustments at the far right of the "track window" to shrink it down enough to see both tracks/bring the video track into sight) there may not be enough room to see both the audio and video track? I'm thinking they are both there, one is just hidden, in this case, the video.
Not saying that that is the answer, but that it could be the issue....