Is the T56 the same as used in the DB7?
Yes, it is the same transmission (The 6 speed manual). The gear ratios vary a bit model to model...
The DB7 GT is geared shorter than the DB7 Vantage, which is geared in a shorter range than the Vanquish S and I think the early models in which I believe you are interested.
So if it is NOT the graziano they are putting in while instead reworking the old transmission, the car will not be as rev-happy as a DBS would (engine is another variable there), but instead would retain it's more midweight GT feel: Lots of torque in the low gears, but with the potential to cross the 200 mile/hour mark. Personally this suits the Vanquish much more.
But we all are sure that the conversion to manual will make the situation better regardless.
The transmission question is still to be clarified...
EDIT:
On a side note just went to a recent thread here, the graziano box as of the DB9 is £9,392.00:
http://www.astonmartinlife.com/forums/db9-gearbox-fault-t1055.html
so if they are not doing much to the dash on the Vanquishes up to 2005, it very well could be the new transmission