When creating a mask with the pen Tool, if you just hover over one of the previously created vertices you get the Convert Vertex Tool.
With the selection Tool (V):
If all the verticies are bold (a little bigger) you can’t see the handles (like when mask shape is selected, the whole mask is selected), so click off to the side, then the verticies (little squares) get smaller and if you click on one of them you get the handles for that one and the ones before and after.
There might be a better way, but it works for me.
Oh, I just tried it using the Convert Vertex Tool and it works the same way, you need to select the vertex, not the whole mask.
You can cycle through the Pen Tools with G.