Yup, this is the one that I think is likely overpowered. I won't know until it's in use but it sure feels roughly equivalent to a key power of a class, like Reckless Attack for a barbarian. Would a barbarian trade no advantage on the first attack in a round but advantage on the rest of their attacks that round for not granting foes advantage to attack them? Seems like a pretty fair trade, in which case that makes Vex roughly the same power level of Reckless Attack. That feels like too much.
I also wonder if this is a hint at the future true strike, changing it to a bonus action to grant advantage. Which would still be slightly weaker than Vex I think.