Okay, I 99% don't believe this statement: it wouldn't be new for AvatarVecna to claim something false at the beginning for whatever reason. Are they a baddie? Are they trying to draw the baddies' attention for tonight? I have no idea, but I see absolutely zero reasons to believe their words or for them to come out as a vigilant at this point of the game if they truly are.
And then the "useful asset" gains trust by simply being useful, people stop questioning their true motives and once the inconvenient truth comes to light it's too late to actually do anything about it.
I'm sorry, but given what happened in the last Upick, I think the "useful asset" argument is not a good reason by itself to keep a potentially dangerous opponent alive.