Vodka dosing makes the bacteria grow like crazy basically. The bacteria use no3 and po4 to grow this is how we get a ULNS. The water becomes so clean that there is no food (no3) for the sps so they starve and pale. Many people have great success with vodka and other carbon dosing methods. However it is a very fine line to having a great looking corals and a super clean tank to have a nutrient stripped tank. In tanks where po4 and no3 are low already vodka dosing is not needed, water changes and a reactor should take care of the rest. In tanks that are heavily stocked or have no3 or po4 issues vodka dosing can be very effective. Try stopping the vodka for a month and see if your corals color up, I bet they will. Or if you feel you must continue vodka dosing then add more fish and/or feed more and dose AA.