When you use a Vector Mask in Photoshop you can use the pen tool to create your path. All you need to do is to close the path and you will have a custom editable vector mask. However, if your mask is white where it should be not be selected and you need to invert it you need to invert the path.
To do this, click the Path Selection tool and target the Vector Mask and click on your path. On the tool options bar you will see an icon called Subtract From Shape Area – click it and the path will be inverted so anything which was masked before will not be now and vice versa.