With the aim of optimizing the interfacial adhesion of basalt fibres with thermoset matrices, in this research the plasma polymerization technique (PECVD) was used to synthesize polymeric coatings based on tetravinylsilane (TVS) or its mixtures with oxygen on the surface of basalt fibres. The successful deposition of the polymer sizing was confirmed by XPS analysis, which highlighted the increase in the intensity of the carbon and oxygen peaks. To evaluate the influence of polymer sizing on interfacial adhesion, basalt fibre/polyester resin composites were tested through the short beam shear (SBS) test. Compared to neat basalt fibres, the modified fibres showed a significant increase in the interlaminar shear strength (ILSS) higher than 180%. These results compared quite favourably with those on glass fibres used as baseline, with higher ILSS values as a function of oxygen content. The improvement in interfacial adhesion was correlated with the increase in basalt fibre surface energy by single fibre dynamic contact angle tests.