I still couldn't find how to do it. Whenever I drag a vertex it moves the whole mask. Tried unchecking "Preserve Constant Vertex and Feather Count when Editing Masks" but it didn't work.
Try following this:
Pay attention to the keystrokes. It's just a matter of clicking in the right place with the right tools and/or modifyer keys selected. Instructions are also in the help files.
Ah, I got it. Just highlight the vertices you need to move.