Most oceanic islands consist of volcanic rocks, mostly basaltic in composition. The assessment of the petrophysical and microstructural properties of erupted rocks is fundamental to define the main factors controlling the explosive character of an eruption. This is particularly important in the presence of a shallow hydrothermal system whose activity may influence the pore pressure conditions of the host volcanic rocks and thus their overpressure conditions and fracturing. These conditions were at the Fogo volcano (S. Miguel Island, Portugal, also known as Água de Pau volcano), where the geothermal reservoir of Ribeira Grande, on the northern flank, is exploited to produce energy. This study includes two types of samples representative of the hydrothermal host rocks of this volcano: massive and vesicular basalts (s.l.). Optical and digital observations (benchtop scanning electron microscope—SEM) are complemented by X-ray microtomography (µCT). The bulk density, unit weight, and porosity measurements are obtained using the buoyancy method suggested by the International Society of Rock Mechanics (ISRM) and also by a digital densimeter. Basalts are porphyritic, hosting plagioclase and clinopyroxene phenocrysts, embedded in a microcrystalline groundmass. The existing vesicles show coalescence and interconnection through microfractures. Massive basalts have a density above 2500 kg/m3, i.e., which is common for rocks erupted at oceanic islands. On the other hand, the vesicular variety demonstrates a significant reduction in its density and unit weight. The results obtained by a digital densimeter closely align with those obtained through the suggested ISRM method, even when employing the instantaneous water immersion method. Some discrepancies in porosity data exist between the ISRM test and X-ray microtomography, probably due to a scale effect, as the regular specimens used for µCT scan are considerably smaller than the irregular lumps analyzed in the ISRM test. Vesicularity appears to reduce the value differences, while compact basalts, if not microfractured, seemingly exclude the influence of cracks that contribute to pore connectivity. All the applied methodologies are non-destructive and should be used to provide complementary information.