Check Airman,
Now I THINK I see what you are asking. You understand that the AoA gauge does not need to know the aircraft weight to tell you your margin from the stall, but you are asking how the computer transposes that information into a forecast speed which it paints onto the speed tape?
Is that your question?
If so, I dunno for sure, but my guess would be that weight of the airplane is still not required. If it knows that the margin is at (for example) 1.3VS at a given moment, and if at that moment your IAS is 130 kts, it is easy to calculate that VS will happen at 100kts.