The Open Group recently announced that they are switching from Prometric to Pearson VUE. For now, the TOGAF 9.1 exam is available at both testing centers (and in many cases, the same testing center supports both testing companies). But at some point in the future, the exams will only be available through Pearson.
The Open Group selects Pearson VUE for global computer-based IT certification testing programme delivery
Source: The Open Group :: Pearson VUE