Capacitor circuit simplification
You simplified the circuit correctly. However, the simplified circuit no longer contains terminals a and b. In your last circuit, you show terminals a and b as being across the voltage source Vi and its series capacitor. From the original circuit, this is clearly not the case. Basically you simplified in the wrong direction.  You need to start on the left side of the circuit and, through the use of capacitive voltage dividers, work your way down to the right.  You should wind up with a voltage source in series with a capacitor at terminals a and b. Since there is no load (i=0), the voltage source (which will be a function of Vi and C) will be equal to Vo.