(I want to say, Cranberry, this is not directed at you in the slightest
)
In general, for most species and size of lionfish that hobbyists are keeping, feeding live feeders - let alone goldfish - is generally unecessary. IME, once established, lionfish can be fed a variety of food, allowing for a diverse diet. And few are particularly finicky about it.
So the concept of soaking or injecting or whatever an asian freshwater carp (aka goldfish) to feed to these lionfish is strange to me to say the least. Again not talking about the exceptions - particularly small individuals, etc like Cranberry works with raising.
Feeder fish do not in any way have a nice life in general. Being soaked in high concentration vitamins would basically cause some very very unpleasant membrane dynamics on a cellular level...injection would be far better, if we are talking about humane treatment.