Vulpes_Inculta: If that's the case, then it doesn't look very useful, since it only works at short range and for 5 seconds.
Still, the description suggests that enemies should think I have disappeared, which means they have spotted me already.
Guarocuya: I can confirm playing a Ventrue run as of the last few days and Brainwipe effectively doesn't work. Whether before or after being noticed.
Do you remember if it was like that with previous patches?
Because when I played as Ventrue many patches ago, brainwipe definitely did work - in the museum mission, if I used it as soon as I was spotted, the guard would immediately forget about it and just stand there going "huh?" or something like that. Then I'd get out of there (sneaking), and find that eventually he'd just go back to patrolling.
I only recall ever using it in that level though, and I'm not sure I ever tried it if they're already shooting - only when they've spotted me but before they start shooting.
It was definitely with the basic patch, since when playing Ventrue with the plus patch I just put them to sleep instead (and therefore never use brainwipe).
And yeah, it's not very useful really - it's best to use it while still sneaking and still undetected, but just before you're (potentially) discovered. That way you can quickly sneak (or run) past someone who would otherwise spot you. It would be just as viable to use the first-level power instead (the one that effectively stuns them), since that lasts longer - except that can attract attention from other NPCs further away, making brainwipe more useful when trying to sneak with a number of enemies around. That and it can potentially affect multiple enemies at once.