You can create modal pop-up without any code in Webflow.
Here is tutorial created by one of our most cheerful Webflowers 
@waldo (only code been used for prevent page from scrolling while modal window is opened)
http://forum.webflow.com/t/tutorial-99-9-webflow-created-modal-tutorial-freebie-clone-able-page/21726
Many of us used this option already and can tell you: this one will work on any device,
Slider can stop autoplay if user touched (“stopped”) the slide. Will try to find more information about this.
About measurements. VH and VW are pretty useful for creating responsive design, which will depend on screen size. But there are always some moments which you have to notice.
For example, in you situation there are no images as elements, only set background to divs. So you can easy use VH. But if there would be images (as elements) inside the slides, you would have to keep slider dimensions in dependency of image ratio, which is easier with Pixels.
Hope I was able to explain