I have a solution here but the downside to it is that the slider images will lose responsiveness.

Idea is to wrap the slider navigation buttons inside a separate div which is absolute positioned and has fixed height lets say 200px, then we need to assign fixed height to the images also which is also 200px. This way it seems that the slider nav buttons are always centered related to the image.

This is just one solution I came up with, still open for other, more responsive solutions.