CAPE TOWN, May 22 (Reuters) - Vivo Energy, a unit of global commodities trader Vitol, will invest an initial 10 billion rand ($550.79 million) in its South African operations as part of public interest commitments following its merger with Engen, South Africa's trade minister Ebrahim Patel said on Wednesday. ($1 = 18.1557 rand) (Reporting by Wendell Roelf; Writing by Bhargav Acharya; Editing by Alexander Winning)