Vivaan isn't currently needed, so he can stay in a coma for the time being. RV-Ragini would've been a classier more psycho pair than RV-Imli.
I think Ranjana twist is related to Vivaan's fate. KN was already trying to kill Vivaan, it'll be silly if he kills his own nephew to avenge Manohar's death. Maybe he'll heart will soften towards him and he'll send him somewhere for treatment, so he can return if the plot needs him.
I agree with you... They should keep his angle open... He can be brought back if script demands...Vivaan and suraj had just recently bonded...I had really liked that part... Hope it is used in the future