You can only book to do Microsoft certification exams through Pearson Vue until the end of August. From then on, you have to choose Prometric. More information can be read at Vue's website: http://vue.com/ms/testtakerfaq.pdf
Personally, I find this a real shame. It's good for us candidates to have a choice. I've preferred Vue over Prometric in recent years, and it was Vue who provided exams at TechEd Australia. I don't know the reasons why Microsoft have taken this approach, but will post again once I have found out something (assuming the information I find out is public).