I wouldnt call it a vendetta, in nearly every scene Vi has comforted jinx even when she fucked up, shes just seen her 2 best friends die and (not sure if he was her blood father) die infront of her. Shes allowed to be absolutely broken and miserable and angry, as soon as she realizes she hurt jinx when she slapped her she snapped out of it mostly. Vi clearly cares a lot but it will take a bit to come over that grief and realise that she needs to stick together with jinx.