First, I’m not sure what an sfap0 file is supposed to be. You need to try to describe the situation a little more clearly.
Based on what you say, you wrote an audio file to disk from Vegas yesterday. How did you write it? I am assuming you rendered it to an audio format which is the only way I know of that Vegas writes out an audio file. You then wrote a video file out today. Again, how did you do that? Did you render a video file, and what format did you use for the render?
Let’s suppose you rendered your audio yesterday to a wav file, and called it “Project A”. Then today, you rendered a video to an mpeg-2 format and called it “Project A”. You would now have two files; Project A.wav and Project A.mpg. In that case, your audio file from yesterday is still intact.
You then start talking about nesting. Exactly what do you mean by nesting. Vegas can nest projects, but it cannot nest audio and video files.
You need to provide a much clearer explanation before anyone can help you.