So I went back to take a screenshot, but perhaps I was wrong? I honestly remember buying the first piece of armor, and the exact same piece was still up for sale.
Anyway, I was able to buy the armor. Thank you guys for the help!!
As the title says, I've done nearly all the achievements for the Decade Armor, I went up to the vendor and bought the first piece. The next piece does not unlock, I believe this is a bug which is why I am posting here. Any help is appreciated.