Viacom Inc (VIAB.O) will allow investors to vote in March on a proposal to extend voting rights to all shareholders, though the measure is certain to fail as it is opposed by executive chairman Sumner Redstone’s holding company. Redstone controls about 80 percent of Viacom’s Class A voting shares through his holding company, National Amusements Inc. Most outsiders hold Viacom’s Class B shares, which do not have voting rights.