The direction of Condemn's knockback is calculated at the point the projectile hits the enemy champion. Condemn's knockback will follow the line Vayne and her target draw at this specific moment.
The duration of Condemn's stun starts when the unit hits the wall, thus the unit can be disabled for up to 2.0 seconds.
Using Condemn on champions that apply suppression such as Warwick or Malzahar is inconsistent. It has been seen on occasion to completely interrupt the suppression, however there have been instances where the enemy is simply moved but the suppression continues, even if he was moved out of the ability's range (e.g. Warwick using Infinite Duress outside of melee range).
Unlike most abilities, Condemn uses a similar mechanic to basic attacks when conducting range calculations - it takes into account the hitboxes of the target unit. For most champions this means an additional range of about 100 units (more pronounced for larger units) and ensures that Vayne can always Condemn if she is capable of basic attacking a target.