eh, where did you get the 20%?Adding thrust vectoring to J-20 wouldn't only add to weight but also to fuel consumption. Together it might cost 20% of range. That's why the configuration of J-20 was chosen not to need thrust vectoring. I suggest it is just as unnecessary as the swing wing.
It's like the first time I read that TVC would reduce the range of an aircraft although it could increse the efficiency for supersonic/supercruise (aka increases the thrust and range) of an aircraft.
The reasons why teh J-20 doesn't have TVC is morelikely the plain lack of a workable system right now.