What did Izaya hire Vorona to do?
Unfortunately, it was Izaya that ordered them to hurt Anri, which happened in the Akane arc.


  Varona arrives at their hideout and Sloan tells her of another job given to them by Izaya Orihara to neutralize, but not kill, Anri Sonohara


As for the reason, it was not mentioned, but it was probably to make sure she would not interfere with Haruna Niekawa as she controlled the Saika children.

Another reason is that Haruna Niekawa wants revenge on Anri and also wants to know the location of Takashi Nasujima. Izaya has promised her both. So it would be logical for Izaya to use Varona to fulfill his promise to Haruna.