Describing Angles

Vertical angles are formed by two intersecting lines, so we should look for intersecting lines on the diagram. One possibility, along with the corresponding vertical angle pairs, is highlighted on the diagram below.

It appears that $\angle DFB$ is obtuse, but can this be guaranteed based on the information given on the diagram? Notice that the marker at $D$ indicates that $\overline{AE}$ is perpendicular to $\overline{DC}.$

This perpendicularity means that triangle $DFE$ is a right triangle, so it can be assumed that $\angle DFE$ is acute. Since $\angle DFE$ and $\angle DFB$ are supplementary angles, this also means that $\angle DFB$ is obtuse. Hence, $\angle CFE$ and $\angle DFB$ are two obtuse vertical angles.