These products do not track the volatility index. They hold a rolling position in volatility-futures, typically blending the front two months to maintain a constant 30-day maturity, which means they inherit contango and lose value steadily in calm markets.
They are designed as short-horizon tools and used as long-horizon ones, which is why long volatility ETPs have some of the worst multi-year charts in existence and have needed repeated reverse splits. Inverse and leveraged versions add daily-rebalancing path dependence on top of that.
Example: the volatility index is flat over a quarter, but the futures curve is in contango the whole time. A long volatility ETP holding rolling front-month exposure loses roughly 5% to 10% a month to the roll alone, and finishes the quarter down sharply with the index unchanged.
Related: etp-roll-decay, volatility-futures, vix-roll-yield, volmageddon