In order to simultaneously account for both R$_{D(}_{∗)}$ and R$_{K (}_{∗)}$ anomalies in B-decays, we consider an extension of the Standard Model by a single vector leptoquark field, and study how one can achieve the required lepton flavour non-universality, starting from a priori universal gauge couplings. While the unitary quark-lepton mixing induced by SU(2)$_{L}$ breaking is insufficient, we find that effectively nonunitary mixings hold the key to simultaneously address the R$_{K (}_{∗)}$ and R$_{D}$(∗) anomalies. As an intermediate step towards various UV-complete models, we show that the mixings of charged leptons with additional vector-like heavy leptons successfully provide a nonunitary framework to explain R$_{K (}_{∗)}$ and R$_{D(}_{∗)}$ . These realisations have a strong impact for electroweak precision observables and for flavour violating ones: isosinglet heavy lepton realisations are already excluded due to excessive contributions to lepton flavour violating Z -decays. Furthermore, in the near future, the expected progress in the sensitivity of charged lepton flavour violation experiments should allow to fully probe this class of vector leptoquark models.