Wall Street’s “fear gauge” has neared all-time lows this year. That hasn’t stopped retail investor Jason Miller from making a nice chunk of change betting it will go even lower.

The Boca Raton, Fla., day trader says he has made $53,000 since the start of the year by effectively shorting the CBOE Volatility Index, nicknamed the VIX. That includes a white-knuckle day on May 17, when the VIX spiked 46% following reports that President Donald Trump had pressured former FBI Director James Comey to drop an investigation into former...