Great question, @Rogier_Schalken! Definitely hope that I can help.
I recently completed a test that is exactly what you are talking about with the native Webflow slider to create a vertical slider.
You can view the project at the following URLs:
Live site: http://vertical-slide-transitions.webflow.io/
Read-only link: Webflow - Vertical Slide Transitions
First, I rotated the slider block 90 degrees so that the transition between slides will be vertical:
But, you will notice when first implementing this style - the slides will be on their sides.
The next step is to rotate a div that is inside of the slide so that the contents are rotated back to normal:
Hopefully this helps, and definitely feel free to reach out if there is anything that I can help with 