In the Pen tool options along the top, set it to Combine Shapes
Draw each corner piece like this to make your work path. Because you chose to combine the shapes, and although this looks like four paths, it will function as one combined path.
Then apply it as a vector mask
Just a quick note here about terminology, just in case you ever have to search for tutorials. This isn't called "cropping". Cropping is when you trim an image down to a different size square/rectangle using the Crop tool. What you are trying to do is called "masking".