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The effect of the refined and acidic vegetable oil content on the catalytic co-hydroprocessing of their mixtures with gasoil was studied in the presence of a NiMo/Al2O3 catalyst. The experiments were carried out in a bench-scale trickle-bed reactor at T = 310–350 °C and total pressure P = 33 bar. The content of the vegetable oils in the liquid feed varied from 0 to 30 wt%. For the determination of the hydrodeoxygenation (HDO) selectivity both gas and liquid phase analyses were conducted. The results obtained by the two approaches are in good agreement and it was estimated that the decarboxylation/decarbonylation pathways were followed by 52 ± 3% of the ester bonds either of the refined or acidic vegetable oil. Decarboxylation cannot be distinguished from decarbonylation mechanism by either method. To estimate the effect of the presence of vegetable oils on the hydrotreater effectiveness, the hydrodesulphurization (HDS) reaction was also studied and the influence of the vegetable oil concentration on gasoil HDS was evaluated. For the NiMo catalyst this effect was found to be negligible. A comparison with the CoMo catalyst’s performance, previously tested, is given.