Presuming you have a lathe, a reasonably close way to indicate a threaded hole is to first turn the head of a screw of the same thread round, then thread the screw into your part. The round head is now what you indicate against.
This method isn't perfect, as you can have thread OD to screw pitch axis errors, but it's generally good to ~.005". If you need better than that there are purpose made threaded gauges that are meant for this use, but they're relatively expensive.
As to indicating in the part, your Rotab has the capacity built-in. Place your part in the vise, then rotate the assembly and watch for deviation of the thread center from Rotab center. Use the X-Y travel to lower the error and repeat. When your thread is at center, you then just use rotation and your indicator to true the vise to X, or whichever way you want to orient it.
Keep in mind that your combination of Rotab and vise is very close in weight to the listed capacity of the machine, so I would suggest not using it for larger work where you'll move the mill table too far from its center. To much overhanging weight will damage both the machine and cutting accuracy.