Why do Solemnities have different readings for the vigil masses?

If Solemnities (like today August 15th) are kind of like extra Sundays, why do the vigil masses have different readings and why don't Saturday evenings have a different set of readings from the Sunday readings? And is there anything notable between the _kinds_ of readings that are in the Vigil Mass vs Mass during the day? I noticed today, for the Assumption, that the Gospel during the day was the whole Visitation, and the Vigil Mass Gospel was almost exactly the opposite (depending on ones reading of the Bible). So they seem to have something to do with each other, but I'd like to know more about the way the readings are chosen for solemnities.