A diet rich in vital nutrients and minerals is always a good idea. It has been shown that vitamin C is a protective, so if I were vaccinating an exclusively breast fed baby (disclosure: I do not vaccinate), I would ensure I am eating foods high in vit C. You can also give it to the baby mixed in breast milk, I would give it in the form of sodium ascorbate.
I really like this series of articles on infant immunity by Hilary Butler, she gives great insight into to the immature immune system and the importance of breast feeding.
"If you find from your own experience that something is a fact and it contradicts what some authority has written down, then you must abandon the authority and base your reasoning on your own findings"~ Leonardo da Vinci
|Selective Vaccination , Delayed Vaccination|