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?
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.)