How does a tiltrotor yaw while in vertical flight?
The V22 Osprey yaws by tilting one rotor backwards from the vertical and one forward by moving the swashplate up on one side and down on the other causing a cyclic change to the rotor blade angles of attack.  This is accomplished using the pedals in just the same way as a helicopter.

Since the rotors are tilted from the vertical, this introduces a horizontal component to the thrust vector.  The rearwards titled rotor produces a horizontal thrust component to the rear and the forwards tilted rotor produces a horizontal thrust component to the front.  This produces a turning moment, using the wings as the lever, around the centre of gravity and therefore the aircraft yaws.

The secondary effects will be as expected.  Since some of the thrust vector is no longer vertical, a little more power will be required to maintain height and I expect that there will be a pitch change although I'm not familiar enough with the aerodynamics of this type to figure out which way and how big.