Even though the Code allows it, be very careful taking credit for internal tray rings as vacuum stiffening rings.  There are several potential problems with this idea.  

You have to meet both UG-29(a) AND UG-29(c) to comply with the code rules for external pressure.  

With internal rings, you will have corrosion acting on 3 surfaces of the ring.  You must subtract the corrosion allowance off of both the upper and lower surfaces, making the ring thinner, and add the corrosion allowance to the inside diameter, making the ring shorter.  Once you do this, you will always have to be inspecting and measuring the rings on subsequent internal inspections to make sure you haven't corroded below the required ring dimensions.  Significant repairs may be required if corrosion exists.  

Code cases are generally used for new vessel fabrication.  As for applying a Code case to a vessel rerate, your local jurisdiction may need to be involved, depending on your local boiler and pressure vessel laws.  In California, Code cases are only allowed on new fabrication and the vessel fabricator must apply to the State for approval to use the desired Code case.  

If your vessel rerate for full vacuum has to be approved by your local jurisdiction, you will probably be better off by attaching external vaccuum rings of the proper dimensions.  

Good luck.