WoW Insider posted last week
about the deactivation of the PvP vendors, specifically those selling conquest gear for honor, thanks to the issues experienced with the Tyrannical Conquest achievement
As the title suggests, the vendors are available again. Senior PvP Developer Brian Holinka took to twitter to announce this:
This troublesome achievement, though, is still causing issues for some players. It seems, from what I can gather, that in order to set the achievement off, you need to earn conquest points. Even just earning a single one will make the game re-count your season total, and award you the achievement. This is still the case even if you have earned well over the required 25,000 points in the season. Just win an arena, or a rated battleground, or do the PvP dailies, and you'll be set.