I have ezacly the same problem in cuore eco 06. Have 155/70/12 dunlops + alloys. earlier when running on stock general tyres it gave the same vibration at 120 kph.
alignment /balancing never solved it. there was very minor alignment/balancing problem when i had it checked so it could not be due to this.
may be its something with this car's light transmission.
its a nuance on motorway the car easily hits 120 on cng but have to bring down to 110 to avoid vibration..
never heard of anyone solving this...seems like a mystery unsolved