Thanks for your quick answer!
I removed the Sprite.
It appears that changing a Sprite-type inside the VBoxContainer to textureRect at least puts the next item in line (a Label) into position right underneath (although I still can't move it "manually").
The Control node in my configuration actually is a child and grand child and grand grand child of several Node2Ds deep within the tree... could this be an issue?
EDIT:
It appears that as children of ControlNodes (or Node2Ds as well) I can move Labels and Buttons etc. around and position them within the VBoxContainer as I like. However, they won't "automatically string" anymore, like when I add text, all the other items to be pushed down accordingly...
(But I wonder why can't I move a Label around as a direct child of and within a VBoxContainer? It seems like I have to choose between "automatically stringing" and "manually positioning" but can't have both, which is a pity really.)