Other threads have said what I've been saying (or similar ideas) on this message board for quite some time now (see below), but in summary, here's my suggestion:

Replace past event item vendors from seasonal events during the current year version (and for good measure, make the achievement available for the recompense vendor, including the Lightning, FFXI, and Dragon Quest crossovers, and THOSE TITLES).

My rationale has been stated before, but I am a hopeless completionist when it comes to these things, and they don't go away. I'm willing to work for those old items, but to not have a chance at them at all is irreconcilable.

AND BEFORE someone comes and preaches about the exclusivity of those things and how it's a REAL achievement... please. Just stop. If you want to discuss your achievements, be on the bleeding edge of PvP and savage modes. I'll even cheer you on! Good luck! You're great!