It is a nice observation. It could hardly be a mistake to spell vat instead of what. It points to uberwald/vampires indeed. I don't know if it is a kind of message to other vampires or undead people but in my blurred memory, the vampires we see in the books in Ankh Morpork are always 'black ribboners' (thank you, I knew the group, forgot the name). A vampire group that is actively 'recruiting' would be new. All in all I think Yuna is probably a vampire, but or me that doesn't straight out point to scum or even cult. Both remain possible but at this time not the most plausible.