Usually a shareholder needs to own more than 25% of the shares to veto major decisions. For Volkswagen there is a special law lowering this requirement to 20%,

Usually a shareholder needs to own more than 25% of the shares to veto major decisions. For Volkswagen there is a special law lowering this requirement to 20%, which conveniently is the number of shares the state of Lower Saxony owns. This law also requires a two-third majority in the board of directors (which is distinct from the executive board) to set-up or close down factories. Half of the members in the board of directors are (as in all large German companies) employee representatives, granting them a veto power for those decisions.