Going by the ovality quoted on Rotor's website (16%) and assuming you arranged the oval rings to maximize the pedal speed at the top and bottom of the stroke (when torque is minimal) you could expect Vector to over-estimate your power by up to 2.56 percent. In reality, the error will likely be slightly less. The error factor will be fairly constant and you could likely substantially reduce that error simply by entering a slightly shorter crank length.
The torque and power calculations used by Vector 3 presume a constant pedal speed through the crank cycle. When this is not the case, the system will either under or over report power depending on how the crank ring is aligned with the pedal arms.