Hi Anja, here are the steps to adding a slider to Vagabond, like on the demo site:
How to add a Featured Image Slider
1. Navigate to Appearance → Customize → Featured Content.
2. Enter the name of a tag.
3. Click the blue “Save” button at the bottom.
Next, if a post you want included in the slider has a featured image, give this post the tag you just added in your “Featured Content” settings. For best results, make sure your image is at least 660px wide.