With the caveat that I strongly disagree with you regarding whether or not to vaccinate your children, you may be able to argue that your moral objection is on par with a religious objection. You also may ask what the state of California defines as a "recognized" religion and thus catch them in a first amendment issue. If you feel strongly enough about it, it might be better to retain a lawyer who will argue your case.
I know that Atheism is not a religion but it seems that all the trendy religionists are saying it at the moment. Of course, it is nonsense but why not toe the line for the sake of income tax relief ? We Atheists can say to Inland Revenue, 'yes, the religionists are right, we are a religion too' and then claim exemption from taxation for all our institutions and activities.
I think I have a smashing idea.
What do you think ?