7 thou" runout sounds a bit excessive to me too... is the table sat square on the bearing, or do you get a height variation as well?
The MT2 adaptor sounds sensible to me, if milled in situ it would be concentric, at least - then once you have it in place, centre-pop both adaptor and table for future installations?
If you wanted to correct the central MT2 socket, you could temporarily loctite the adaptor in place for milling and set-up and then heat the adaptor until the loctite lets go once indicated in (not incredibly hot - might be a good idea to put a central tapped hole in the adaptor so you can jack it out with a tube and bolt)
If you took the rotary table to the lathe face-plate, assuming it's big enough - otherwise dismantle it to the point where you can get it on the lathe? The table should unbolt from the wormwheel (I assume the Vertex is what Soba cloned, and the Soba's wheel does!) and could be bolted to a flanged stub mandrel to go in the 4-jaw if you don't fancy the idea of the complete rotary table whizzing round while you bore the central socket

I expect your lathe could handle a 6" disc!
Once on the faceplate / in the 4-jaw and indicated on the newly-machined reference, you could then rebore the taper and follow it up with a MT2 reamer if you're lucky enough to have one - it would be hard to make the runout worse than it is!
I've found some of the far-eastern rotaries can be very accurate, sometimes the same manufacturer gets it wrong, though - I have a 4" tilting table where the tilt pin isn't parallel to *any* other surface, by a few degrees - eventually (tuits permitting) I'll put it on the lathe's boring table and - well - bore it straight! Apart from the tilted pivot (and the crude angle setting) it's pretty accurate, well under a thou" variation in height , bore centre as the table rotates etc. - I was thinking I'd put another mod on it to make it a "sine rotary table" on the tilting axis while I've got it apart? That would mean drilling and boring for another pin parallel with the pivot and an accurate distance away - relatively easy while the table's bolted to the lathe!
Just my ape knee's woof,
Dave H. (the other one)