How is a sideslip maintained (aerodynamically)?
(Source)

Here's a representative image of the variation in lift coefficient with respect to angle of attack.  The same airfoil is considered in a "clean" configuration, with a deflected flap, and then with both flap and slat (you can ignore the last, which is the uppermost example).  You can see that, with deflected flap (or, shall we say, rudder) the lift slope shifts upwards, changing the lifting generated at a given angle of attack since the rudder deflection has, effectively, changed the airfoil's camber.

Hence, while the airfoil's angle of attack has changed, the lift generated at that angle of attack has also changed.  Cambered airfoils can generate lift at negative angles of attack and so, for a given rudder input, the aircraft will assume a steady sideslip angle where the induced yawing moment is in equilibrium with any restoring moments generated by the rest of the airframe.  From my time flying gliders, this could be illustrated by the fact that, no matter how much rudder I put in, I could only make the aircraft yaw so far--there just simply wasn't enough aerodynamic control authority to overcome the force you mention as well as unbalanced yawing moments generated by the fuselage and wings.  But, at the same time, I could intuitively change my sideslip angle by proportionally varying the amount of rudder I put in or, to be consistent, I could modify the aircraft's equilibrium position in yaw by incrementally "changing the camber" of the vertical tail.