If the J-20 gets TVC I suspect it will be more for high altitude maneuverability, since the effectiveness of control surfaces drop in thinner atmosphere.Put TVC on WS-13 for J-31 makes more sense as it is more useful in a conventional aircraft. WS-15 on J-20 may not need TVC at all (if PLAF want to use every drop of the thrust), but a bonus if post-stall performance is desired.