Hi-
Although we don't usually call it a "slipped vertebra", if you do indeed have a fractured pars (a part of the vertebra which does in fact help it stay in place), then one vertebra can slip on the one below. This is called a spondylolisthesis- quite a mouthful! Although it is possible to treat this non-surgically, occasionally patients do need surgery for this condition. Chiropractic care is a good route of treatment, if it is helping. You should feel results by the end of 1-2 weeks of treatment. If not, then your chiropractor should be considering other treatment routes- either changing his approach, or referring to another practitioner.