The current pre-release of the client ("pioneer" in the version) is only compatible to itself. So you can only play with other testers. Please be aware!
Access to the replay vault is equally open to everyone. Maybe they changed their name? Remember that you can only search replays going back 1 year unless you change the date range in the search options.
Look at your own replays to find a match you played with them, and the replay vault should show you their new names.
