Making sauerkraut with fish sauce instead of salt?
Salt is needed to give the good bacteria an edge over bad bacteria that do not handle salt so well. The amount of salt necessary differs from maker to maker. While the salt does draw the moisture out of the cabbage, you can also make tasty sauerkraut in a typical fermented pickle manner - shred your cabbage, pack it in a jar, pour salty water over it and let it ferment. Initially the texture will be more "pickled cabbage" than traditional "sauerkraut", but if you leave it long enough it yields the softer texture. Now if you don't want to use the salt plus water try fish sauce plus water.
  A frequent concentration for pickling vegetables is one tablespoon of (kosher) salt in two cups of water. Make some up and taste it. Then see what mixture of the fish sauce and water has a similar salty taste level. Pour that mixture over the shredded cabbage. Let us know in a few weeks if you survive!