That would be some serious seal steam leakage to drive your unit past the turning gear over-running clutch. The turbine will always spin down over a greater time with vacuum as there is no windage add mass and the rotor will slow (very simple). If it is mandatory to stop the turbine then secure seal steam / break vacuum. If you cannot get the shaft to stop and you have verfied steam shut off to the unit and HRSGs on complete bypass to condensor, no need to worry. This is natural and will take its time to come down, I do not suggest forcing the unit to come down to turning gear speed unless there are issues requiring a stopped shaft.

How much time are you taking to come down to gear w/vacuum? Once in gear is your clutch engaged properly or bumping in and out due to mechanical issues? What make is your turning gear Koenig or Nuttall? Have you tested the gear motor rpm, amps etc..

How do you maintain vacuum, pumps or ejectors? If your D11 is shutting down do you have an aux steam header supply for the seal steam, ejectors? Otherwise you have to break vacuum and the shaft is going to go to gear.