I think the other lady (Lady Marlotte-or something) was hinting that She and Vetinari had some kind of an affair/relationship- or at least thats how I always read it. I also believed that it was he that turned her off human blood. We know that vampires need a new obcession to give up the blood and I think he showed her that she can get the power from politics not blood.

Some characters stay and some go, probably becuase some are more interesting tp pratchett or they can more places to grow/ways to be used.