It's all to do with angle of attack of the wing section to the direction of airflow.
In a fully symetrical wing with zero AoA, lift is zero. It therefore needs to be angled slightly relative to the direction of airflow in order to produce lift.
In a more typical wing section, or airfoil, the higher curvature of the upper surface will produce some lift at zero AoA.
So in your scenario where the aircraft is in unaccelerated vertical flight, for the (non symetrical) airfoil to produce no lift, it will need a slightly negative AoA.