Just a chance here, but have you looked at a stereo version of the file?
Sometimes, if your very lucky, the vocals are recorded on just 1 channell. If your even luckier there isn't much else on the track with the vocals.
Long shot, but with older recordings its deffinately worth a look.
Cant remember the name of the album, but as a vague example I had a 'Rolling Stones' vinyl where all that was on the left track was the vocals.