The changes I make to the location and rotation of the virtual bones I create are not saving. In other words, I make my changes and click save all, close the skeleton persona, reopen it, and my changes are not saved. Why?
I am 95% sure, that is the way it is intended to be.
The only data you are supposed to set is the start and end bone.
The virtual bone pretty much always points from your start bone to the end bone, and is also live updated when animations are played.
Epic posted a video showing how the bone is updating in realtime on twitter: https://twitter.com/unrealengine/status/805396522422702080
You can use virtual bones as just like normal bones inside the animation blueprint. I played around with it for a little while, but failed to come up with a good example when this whould be usefull.
Hope this answer somewhat helps.