Polyhedral oligomeric silsesquioxanes (POSSs) have received great attention owing to their structural stability and diverse external functional groups. The extensive development of synthetic strategies has yielded mono- and multi-functionalized POSSs. The synthesis of functionalized POSSs, however, generally requires complex reactions and tedious workup processes; therefore, it is difficult to meticulously control the structure of POSSs. To address this problem, we propose a novel straightforward and universal strategy to produce multifunctional POSSs with the desired number of vinyl groups. In brief, POSSs were built through cohydrolysis-cocondensation by employing methyltrimethoxysilane and vinyltrimethoxysilane (ViM-POSS) in a one-pot system. 1H NMR and X-ray photoelectron spectroscopy reveal that the stoichiometric ratio of the vinyl and methyl groups in ViM-POSSs is almost identical to the theoretical value. Moreover, all ViM-POSSs are synthesized as T-8 POSSs with rhombohedral structures in the R-3 space group and are stacked in the ABC sequence. Furthermore, as the portion of the vinyl group in ViMPOSSs increases from 0 to 100%, average molecular size, average molecular weight, and sublimation temperature of ViM-POSSs go up steadily. We therefore believe that this facile and effectual synthetic route will set a precedent for POSS synthesis in both academia and industry.