From what I understand, the general consensus is that nursing moms SHOULD continue taking their prenatal vitamin while nursing. It helps make sure the mom is getting plenty of nutrients while her body is making milk. If the mom is severely anemic, she won't produce milk because her body will focus on building up the blood supply
(I have a friend who is a homebirther and die-hard extended BFer, and after one of her babes was born she had to feed with a SNS because she lost a lot of blood, borderline to the level where they give transfusions, got anemic, and didn't make milk until her body had the blood situation under control!)
I take a multivitamin anyways, pregnant, BFing, or not.
Kathryn