It kind of made sense to me:
1) Icarax wears the MoS
2) Vultraz is Icarax's sidekick
3) Icarax discards the MoS and wears the Kraahkan during the Karda Nui arc
4) Vultraz wears a MoS during the Karda Nui arc
Vultraz's mask was likely linked to Icarax because 1. It lacked a name and power, 2. Icarax's former Mask was unknown and 3. Both characters/sets happened to look very red and very evil.
I mean, I would prefer it if it were the way I understood it (which is nothing more headcanon at this point), but I know it's probably going to be yet another case of "The Matoran just happens to wear a Noble version of his/her Phantoka counterpart", which is something I've always disliked.