Are you using your lowest aimpoint (the bottom 'arrowhead' in the reticule) when you shoot at 50 or the crosshair? 50 yards is a poke with a Vix, I'd expect some 30 inches of drop. I'm using the bottom 'head to hit 50 with my Vortex!
So either aim higher or turn you speed ring down (doing so increases the distance between aimpoints).
BTW, the numbers on the speed ring are just guides, don't take them at face value. Keep lowering your speed until you hit at 50; although 35 yards may be more practical distance with this bow.