I don't know if Vagary is just different or whatever, but that's not traditionally how TH works.It won't, a thief has a chance to increase the amount of slots items can drop in, but they won't help you "force" the platemail to fill one.
It's all random luck.
TH generally doesn't add "drop slots", as pretty much anyone who has extensively run something like Neo-Dynamis can attest. (What WILL add drop slots, in Neo-Dynamis, is getting procs on the mobs.) TH just makes it more likely that those slots will be filled.