Yes, you will always need a valid e-vignette unless you avoid toll roads entirely.
Additionally to mobile units, there are cameras scanning license plates on every toll road.
As soon as your vehicle plate is scanned and a violation is found you will receive a heavy fine, unless you purchase an e-vignette on the same day.
You can order your Slovak e-vignette comfortably here.
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Can I drive to the first exit on the highway in Slovakia without a valid e-vignette?
What happens if I drive without a Slovak e-vignette?
Which roads require a Slovak e-vignette?