isolate the side with heavy vocals (copy and paste to a mono wav file if you can)
play with eq until it is mostly all vocals
copy the vocal only channel and mix with the oposite side.
if you have multitrack software this is pretty easy to get the right mix, if not it is trial and error
Make sure the new mixed side didn't overdrive. If it did you need to lower the level of both sources and try again
normalize the original vocal heavy side and the new mixed side individually
use the original vocal-heavy side and the new other side to make a new stereo wav
listen to see if you got it right.
if not, then start all over
I think this is all you can do. It will most likely screw up the immage, but there isn't much else you can do other than making the whole recording mono
Best thing to do is to record from the center of the soundstage.
Good luck!
Matt