The X/Y problem is that Logic only allow to send one parameter to VSL, so you use the velocity of the articulation to define the Y value.
You are right it is not 127, it is 254 articulations per instrument
With Levell II patch you have less than 50 articulations per type of instrument.
i.e. Violin, Viola and cello have the same articulations, Double bass have it's own set
You say : "I agree with you that Logic may never adopt AU3". this is false
Since the 4 march 2017 : "The announcement says that Logic 10.3.1 supports AU3 and multi port midi "
This mean that VSL need to change there code