Vistra down on Q2 profit fall

** Shares of U.S. utility Vistra Corp VST.N down about 6% at $189.01 premarket

** Company posts lower Q2 profit on higher operating and interest costs

** VST posts Q2 net income of $327 million compared with $467 million, a year ago

** Company's interest costs rose 25.7% to $303 million in Q2, while total operating expenses increased about 17% to $733 million

** As of last close, VST shares up 45.6% YTD