I have seen her do crazier and meaner things. If no one was looking, it would have been super easy to grab it- the cameras actually aren't on them at all times. And if it WAS a set-up by production, I don't see Vivica agreeing to it- when there is something about you you're extremely sensitive about- say, your aging beauty- you do everything you can to keep it from being talked about or brought up. And Kenya is delusional enough to think that she'd never get caught, and she never worries about the repercussions of ANY of her actions. She probably didn't have time to think it through, actually- saw the opportunity, grabbed it, tried to humiliate Vivica.
I don't see any other possibilities, really. Kenya has gone HARD in RHoA reunions when someone has made a false allegation against her, but she gets that exact same smirk every time she's called out on something shady she's done. She loves to deny and smirk, knowing that they know but can't prove it.
My biggest thing? The fact that she knew the tweet EXISTED AT ALL, with her "I just started following Vivica a few days ago..." riiiiight.