I notice that Hisako's Vengeance Counter stops both high and low attacks, as if she has instinct. Is this too strong, or maybe a bug?
Yeah, during instinct, Vengeance will counter highs and lows. It's definitely intentional. I don't think it's too strong, either... it's just a short time where she gets to cover a few specific situations that she otherwise couldn't. Not too uncommon for instinct modes to do that.