The selected canvas layout is visible in the right corner. Based on the selected layout, the slide detection and split video algorithm will work.
In case it is not visible, canvas is disabled in advanced settings
In the advanced settings you can select a canvas layout, however, you need to make sure you also adjust your vmix canvas to match the slidedetection/split video canvas.
You can quickly change the slide source with alt+click on the layout icon.
For vMix templates see vMix templates