Depends what you mean by a healthy immune system. I mean, if you had a very unhealthy immune system then vaccines are useless, and if you had a super healthy immune system where all possible diseases are already accounted for, then you are immune to disease. Never-the-less lets just say healthy immune system is the healthiest immune system a human can get naturally, while the immune system with vaccines is the immune system of a relatively unhealthy person, then I would pick vaccines.
Psychology or Neuroscience?