I too did Vodka and Prodibio, but for PO4. Neither worked.
A fuge would be my first recommendation. Before I installed my fuge my nitrates would run around 10. After I put it in I never had a problem with nitrate at all. I use a moddified Octopus NW 200, with 110 lbs. LR and a DSB. The DSB is about to come out though as I know it is giving off nutrients.
It is possible your LR is saturated with nutrients and may need to be cooked. My LR is 5 years old, and I know it is saturated with P04 and nutrients, mainly from my sand bed. If you have hair algae or bryopsis on your LR and are measuring 0 for PO4, your LR is saturated with PO4 and other nutrients. The hair algae takes it up first off the LR before the nutrients have a chance to settle into the water column. Both my PO4 and NO3 read 0, but there is obviously nutrients in my system for the algae to grow. My chaeto has almost all died now though. But my bryopsis is growing very fast-a definite indication of saturated LR.
Tonight I begin the process of cooking my LR. I'm going BB in both my DT and fuge after the process of cooking all my LR if finally complete in a 2-3 months.