If that rock is dead old "used" live rock but was not cleaned id do a HCL (Muratic acid) bath with it first to clean it up like new in box dry rock. You would want to dilute the HCL with water, sit a few days then rinse it with clean water and making the final rinse with RO or distilled water. Then when your certain its squeaky clean & rinsed then place it in the tank. This way no die off to pollute tank & grow bacteria & algae blooms out of control minimizing your cycle time.
Far as placement before or after depends on the live stock you plan to keep, if anything that is burrowing is planned like a Goby or Pistol shrimp etc.. then its best to glue the rocks down or place them on a small grate sitting on the glass then place them, finally placing substrate in after so no chance of the rocks getting tunneled under causing rocks to shift or worse drop or fall causing damage.
Vinegar is best used for cleaning off calcium & encrusting coralline algae on used equipment when doing matinance. The HCL would clean a rock like new again much better then vinegar, just work with it outside & use safety glasses & rubber gloves.