Playing with V1.5.14 S3 (64-bit) and Frozen Fangs expansion only. Had been playing random single player games for a while, and don't recall ever playing as the Mezari. Thought it was bad luck, but then went through the list to make sure I'd played every faction available to me. Chose Mezari, and it loaded as Vaulters. I tried a couple times to make sure I hadn't picked Vaulters by mistake. So you've got to have a variable set up wrong that's pointing to Vaulters instead of Mezari. It's the only way it makes sense for it to mess up on random and manual selection. And I really think I'd had an unnatural numbers of games as the Vaulters when playing on random that supports the idea that it fails in both ways. Let me know if I have some log file that would help with finding the bug.

Could we get a save file from you, along with a screenshot from your military screen? Select any Mezari/Vaulter unit design in there and just press F12.