I personally use Reptivite by ZooMed. No specific reason, would be open to using another brand if a better one was recommended.
I do dissolve it in RO water, with GSE, and inject it into the food item though. Make sure you use a large needle as it doesn't completely dissolve. I think I have a 12 gauge needle (does that sound right Hurley or CornCrazy? I know it's a needle that my boss uses for horse injections.). It's nothing really scientific either, as I dissolve a clump of vitamin in a splash or two of water with a 1/2 drop of GSE. I inject about a 1/2 cc into smaller prey (pinkies and fuzzies) or less if they feel as though they are getting too large. I use about 1 to 1 1/2 cc's for hoppers and mice.
I give either vitamins or Nutri Bac this way approximately every two months alternated.
D80