This probably won't work since you say that at least one of the files was an .mp3. In that case you cannot possibly get back identical files because too much audio was thrown away when the original was compressed to .mp3. Also if both are .mp3s there is even less chance because the encoding of the track including vocals will be different from that of the instrumental track.
But there is no harm in trying to see if it will work but I very much doubt it.