In the current study, pre-composting and subsequent vermicomposting of separated digestate (also called separated biogas plant slurry) were verified to improve its properties for use as a soil conditioner. Increasing the proportion of straw in combination with separated digestate had a positive effect on reducing both weight and volume during vermicomposting. The pH value decreased and was found to be between 7.3 and 7.7 in the final vermicomposts. Due to a loss in organic matter, the total content of macro-elements increased at the end of vermicomposting with the exception of calcium. Similarly, the available contents of phosphorus and potassium increased, which was positive in terms of the use of vermicompost for agricultural purposes. After five months of vermicomposting the earthworm biomass increased from 282% to 896% depending on the treatment. The best results were achieved during the vermicomposting of a mixture of straw (25% vol.) with separated digestate (75% vol.).