how to answer "Where did you draw the line on what you included in your literature review?" during the phd viva?
One simple answer is (leave away those aspects that don't apply to your situation):

First, I systematically thought about the different directions in which related work may exist. To make sure I did not miss any relevant directions, I consulted with my advisor/other researchers and checked the related work of the related work I initially looked at. For each identified direction, I considered a handful of prominent papers, until a point was reached where new papers did not add substantially new ideas, but just variations of the ones I already had covered.