I'm in VA, and AFAIK, it's all or nothing b/c we only have medical or religious exemptions. At any point you can decide to "change religions" and stop vaxing, but that means all vaxes are stopped. If she claims a religious exemption and then at a later time vaxes for 1 or 2 VPDs, the state could compel her to bring ALL vaxes up to date.
We decided when DS1 was 12mo that we would no longer be vaxing. We have not vaxed him or any other children since that point.
ETA~ here is the accepted exemption form:http://www.vdh.state.va.us/epidemiol...ents/cre_1.pdf
Because the state does not ask for "proof" of religion, a person can change their religion at any time and stop vaxing.