It doesn't track spot vix. According to the prospectus, it tracks spot vix PLUS OR MINUS a premium or discount. You can calculate this premium (discount) on their NAV page daily.
Because VIX was pretty low at launch, a lot more investors were betting VXUP, meaning VXDN got a steep discount. That number was nearly 25% discount for ONE MONTH! You'd have to be right by a lot to win with this vehicle.