In phase 3 it was figured out the the stat caps on items were the stats that would be shown on the HQ versions, so if you had something with str +5 and the HQ version had str +7, the str cap on that item was +7, so you had to chose something different than str to meld on the HQ item.
I'm not sure whether this is still the case since I ran across a player wearing full weaver AF in phase 4 with triple melded craftsmanship materia all through.
I'm wondering if you would be able to meld intelligence materia on HQ vanya clothing, or strength materia on HQ heavy dark steel armor.
I'm personally considering either acquiring the dark light cowl of casting using my cracked melded gear, but I'm wondering if melding the vanya hat and robe would prove a more powerful alternative.