This study examines the effect of three key operating factors (peak temperature, particle size and pressure) on the potential stability of the biochar produced by slow pyrolysis of vine shoots. The following response variables were considered as key indicators of the potential stability of biochar in soils: the fixed-carbon yield, the fraction of aromatic carbon, and the molar H:C and O:C ratios. Slow pyrolysis tests were conducted in a laboratory-scale fixed-bed unit and planned according to a 2-level factorial design. The behavior of the product gas yield and composition at the outlet of the secondary cracking reactor (a fixed-bed of activated alumina particles at 700 °C) was also evaluated as a function of the three factors. The results from the statistical tests revealed that the particle size is the most significant factor in determining the potential stability of biochars. Using larger particles of biomass and, in a lesser extent, operating at higher peak temperatures leads to the production of more stable materials. Unexpectedly, the absolute pressure only plays a significantly positive role in decreasing the tar content in the producer gas at the outlet of a secondary cracking reactor.