Vanadium oxides are among the most promising materials that can be used as electrodes in multivalent rechargeable batteries. Specifically, they can be used as cathodes in Al ion batteries, but voltages reported so far are low, limiting energy density. We use amorphization as a strategy to increase voltage and compare the energetics of Al insertion in crystalline and amorphous vanadium dioxide (VO2). We start by developing and optimizing the force-field model for vanadium oxide phases, which are then used in generating the amorphous structure of VO2. The Al insertion sites in both amorphous and crystalline phases are then systematically investigated by first-principles calculations. We show that Al insertion in amorphous VO2 can occur with well-dispersed insertion energies, with a lowest energy site more thermodynamically favourable than any insertion site in the crystalline VO2. We compute the voltage-composition profile for amorphous VO2 and show that amorphization increases the average voltage of the VO2 cathode by about 0.85 V, as compared to crystalline material. We also suggest that the stability of the amorphous VO2 cathode is improved by reducing volume expansion during Al insertion, potentially leading to longer cycle life in practical applications. Overall, the demonstrated improvements suggest a high potential of amorphous VO2 electrodes for multivalent batteries.