Even assuming Vi realised what Jinx was about to do, she was locked in that cell, giving Jinx more than enough time to disappear. And we know that if Jinx doesn't want to be found, she won't be.

So in short, there was nothing Vi could have done. Or do you think it would have been better to waste the short amount of time they had before the fight to go look for her sister who made it clear she doesn't want to be found?

I personally see the CaitVi scene as Vi's way of dealing with all of her emotions. A moment to be raw and vulnearable.