I think you are on the right track...they need N...have you any dolomite lime available? If so it is a very good buffer and helps keep the soil around 7 or below...(at this point you'll either have to re-pot and mix it in the new soil or mix it in with the water) I don't have as high a ph problem as you, but you might try some other acidic substances to help get the soil itself lower in ph, like citrous zest (grind up the peels), coffee, and I use organic lemon juice sometimes, and there's others...mix them up and apply different ones instead of just the vinegar maybe, (like one time use coffee, the next use lemon juice, the next something else and watch the soil ph to see what it does) but you need to get the soil itself lower in ph so the babies can suck up the nitrogen your giving them...also maybe try some unsulphured blackstrap molasses (no more than a tablespoon per gal)...it can help the roots get more nutes, but does little to the ph. I don't think it's that your not giving them enough N, it's that the ph is just too high for them be able to utilize it. Hope this helps, G/L!