Vansh had a major drug abuse history behind him and was still not totally rehabilitated... Tap Ni took advantage of his weak emotional state to to create trouble between Vancha n finally taps even instigated him to kill himself... Vansh wud have never done that to anybody ...so please dont compare Vansh with that evil taps...
Im not making any comparisions merely highlighting that if Vansh could be poisoned by Tapasya as an adult within two meetings then Tapasya as a vulnerable child would be much more susceotible to poison. And please dont forget that Vansh had a gun and was not only pointing a gun at Tapu's head but was going to kill your beloved veer and icha too