What kind of flow do you have in the tank?
Do you have any sponges or other filter media anywhere on the system? Check in your sump for collected detritus. When I was battling high nitrates if turned out that I had a dead spot in my sump and was loaded with detritus. After correcting that issue my nitrates run around 10ish.
BTW to get the full benefit of dosing vodka you have to do it for a while. It doesn't happen overnight. I'm not a fan of adding sugar to the tank though it has proven benefits. As you found though it can mess with other parameters.
Do you have a TDS
meter on your RO
unit? Have you tested your source water before the RO
and then tested the pure water to see if there are nitrates there?