Obsessions are fine- provided that they are taken in moderation. Unfortunately, Clan Vruzz seems to have ignored this good rule of thumb. At first glance, they don't seem like anything special. However, there are two details that set this clan apart from the rank-and-file:
1. They enjoy poison.
2. They are completely BATSHIT insane (even by Skaven standards)!
With these details in hand, it can be inferred they are thralls to either Clan Eshin (who does use poison for certain assassination jobs) or Clan Pestilens (making others ill is their thing, so...). They could be both or neither; with such little information on them, it's hard to say.