WT777,
The yaw-slip thread on this forum contains the answer you look for in (some would say) painful detail.
Briefly however, a body subjected to a rotational force, such as yawing, will tend to rotate about its own centre of gravity.
An aircraft yawing, due to asymmetric thrust, will tend to yaw about its own normal axis, which passes through the centre of gravity.
To stop this yaw, you will need a moment exerted to oppose the yaw. Moments are equal to the product of a force and its arm (distance between the force and the centre of rotation).
In the case of the aircraft yawing due to asymmetric thrust, the rudder produces a force. The anti-yaw moment is equal to this force multiplied by the distance between the rudder and the c.g.
To prevent yaw, you can either have a rudder displaced a little bit (small force) a very long way from the c.g. (long arm), or you can have a rudder fully displaced (big force) a short distance from the c.g. (short arm).
To oppose yaw, an aircraft with an aft c.g. (small arm) needs the rudder displaced more, than another aircraft in the same situation but with a forward c.g. (larger arm, therefore smaller force required from rudder).
If both aircraft decelerate together, guess which one will run out of rudder authority first? Yup, the one which has the shortest arm (aft c.g.). This is the one which will have the highest Vmc.
No idea about packs on a 757 though...
cheers,
O8