I own Assasins Creed 3 Liberation and have played it for hours, I have yet to get the (chain kill ) funtion to work, it just doesn't work at all. To get it to work, you press (right) on the d-pad on the vita. this function/ action don't work on mine. Was my game possibly defective or am I not doing something right ??
It may be a different button (I always forget), but I'm sure it's on the D-pad. Just button mash the D-pad when in combat and see if that works.
Note though that it's annoying as crap to use. You have to select enemies with the touch screen, but it doesn't always work on the first (or even fifth) touch.