This achievement required a hotfix back in patch 5.2, too, as it was incorrectly calculating the points required. It seems that, with patch 5.3, things have gone from bad to worse. The reason this is an issue is that the achievement's completion, at 25,000 conquest, now allows players to purchase conquest gear with honor. Allowing players to purchase that gear with honor at a far lower points earned level than intended was clearly a bit of an issue! Hopefully the developers will be able to implement a fix very soon, so that PvPers can continue to purchase their gear.
Update: Senior PvP Designer Brian Holinka tweeted WoW Insider this morning to clarify that the only disabled vendor is the one selling Tyrannical gear for honor. The Tyrannical conquest vendor is, in fact, still available.