If the MACD was chopping around the zero line as soon as the market sold off to initiate a VTO buy the MACD histogram would go negative on the weekly blocking the buy. Furthermore during the time the SPX MACD histogram was below zero last year (blue rectangle on chart) many good VTO buys were missed. But one particularly bad trade in August was prevented.
Therefore I have decided to lower my RSI buy line to 8 when the weekly SPX MACD is below zero, thereby eliminating many trades but preventing any losses. Upon further reflection it appears to me to be prudent to focus on short swing trades when the weekly SPX MACD histogram is below zero and long swing trades when above.