The comma-separated solution works for built in (3D) vectors, but apparently not for the built-in (but defined as a struct) Vector2D. There I had to set the default values as:
(x=1,y=1)
Both the parentheses and the x= and y= are important.
Also it seems, once you set a default value, you can’t clear it any more, you have to enter something. Maybe this has to do with undefined behaviour for already instanced functions. One way to clear the value for a Vector2D is to use ().