What some people call the "vertex angle" refers to what you see in the triangular cross-section; the Fresnel biprism has a very flat isosceles triangular cross-section.

If you think of a prism as a 3D polyhedron the term is misleading, as the vertex of the cross section corresponds to a long edge of the polyhedron. In a Fresnel biprism the internal angle along the middle edge is close to 180 degrees. This shows in the cross-section as the very flat apex.

In practice the very sharp edges on either side are often cut off or truncated, so what you get is a similarly flat pentagonal prism, as in the sketch below.

[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/0Olvo.png