AMD ReLive creates mp4a files when I capture using multiple mics... they may open fine in v14 but I find I have to rename them with an mp4 file-type to get later versions of Vegas to recognise them.
mp4a files, can often be HE-AAC, and it requires the so4 decoder that came out with VP15. I experienced this problem only recently with my YouTube download containing HE-AAC, I normally edit using legacy AVC (same as VP14) and doesn't load, turned off legacy AVC, and it loaded. A warning though, vegas appears to be much more unstable with HE-AAC with a huge amount of crashing so VP15+ may not be the answer either. Not used them enough to verify that though. might be easier to re-encode the audio track