dustbug - vestib and medical audiology are where you ought to develop solid foundation. as far as hearing aids and cochlear implants, know the basics, but you learn most from the job and the feedback your patients give you (no need to remember RETSPL numbers and all that stuff). certainly, know the test protocols and why/when they should be used.
steph - private practice nowadays pretty much narrows it down to hearing aids and hearing aid evals. there are some private practices that perform vestibular testing, but there aren't many that would do just diagnostics and no hearing aids because that's not where the money is. while we're not here just for the money (hopefully, because we'll be pretty disappointed if we are), we still need to put bread on our tables too. you'll be surprised by how little you make per hearing test, especially through health insurance or medicaid/medicare. if you're into diagnostics, try a hospital or ENT clinic. hospital settings tend to get all the crazies but little hearing aid experience. ENT clinics tend to get middle ear pathologies and some hearing aid experience. i hope this helps. 🙂