A series of vanadium complexes bearing dianionic tetradentate amine-bisphenolate [ONNO] ligands, V(O)X[ONNO^Me] (X = Cl (1); OⁱPr (2)), VCl₂[ONNO^Me] (3) and V(OⁱPr)₂[ONNO^R] (R = Me (4), ^tBu (5), Cl (6)), displaying various electronic and steric properties have been prepared. The molecular structures of two of these complexes, namely V[ONNO^R](OⁱPr)₂ with R = ^tBu (5) or Cl (6), are reported. Activated with dialkylmagnesium, all complexes lead to modest isoprene homo-polymerization activities at 50 °C. Quantitative polymerizations were observed using 4 and 6 as pre-catalysts combined with AlⁱBu₃. The resulting polyisoprene microstructure was composed of ca. 70% 3,4 enchainments, the remaining 30% 1,4 enchainments being a mixture of cis and trans stereoisomers. 6 leads to a more active catalyst than 4. β-Hydride abstraction occurs during the reaction.