Can I set the Full page sliders to use vh or % instead of px?

The Full Page responsive mode has a Modify slider height feature which you can use to exclude certain element's height from your slider's.

The trick you could do is, that you could make an element, which wouldn't be visible, it would have an absolute or fixed position with some low z-index, it wouldn't have anything inside it, just make it to have the percentage height you want to remove. 

1
Setting the slider to have % height
Create a new, empty div, like:
<div id="slider-helper" style="position:fixed; top:0; bottom:66%; z-index:-100;"></div>        

Write #slider-helper into the "Decrease slider height with the height of the matching elements (CSS selector)" field, which would make the slider remove the height of this element, and the slider would have 34% height.

2
Setting the slider to have vh height
Create a new, empty div, like:
<div id="slider-helper" style="position:fixed; top:0; bottom:74vh; z-index:-100;"></div>        

Write #slider-helper into the "Decrease slider height with the height of the matching elements (CSS selector)" field, which would make the slider remove this, and it would have 74vh height.