oopsylon says:
... would blood bags count as ‘feeding on someone’ ...
Ultimately it is up to you how you want your feeding to work. Do what is most fun.
Blood bags
could count as 'feeding', but probably not 'on someone'. Stored blood could be mixed and not from just one person. Standard medical ones also contain other chemicals —to stop the blood from coagulating and so forth— so might not be ideal.
As you say, 'they' obviously can not suffer harm, and you probably don't get to learn something from 'them', and a miss makes no sense. That means that you don't roll (the Trigger is not met if all the parts don't fit the fiction).
The purpose of the move is to add drama, eating a 'microwave dinner' is not dramatic, so the drama needs to come from elsewhere. There is a reason your blood-supplier incurs Debts for the task, it is not trivial to get, the drama comes from the challenge of acquiring the blood. If this requires us to rethink who your supplier is (maybe an NPC who you don't ask too many questions of?) then we can revisit that Starting Debt, but maybe
Jacob is the one not asking too many questions about the 'willing' people he sends your way?
How much you can get from stored blood would depend on the fiction at the time. Where did it come from? How fresh is it? How desperate are you?
It is also possible that feeding from stored blood merely slakes some of your hunger —takes the edge off— but does not provide the other benefits, both the secrets and the healing seem to stem from the vital intimacy of the Move, no?
If it is easy to just grab a few bags and heal up, that detracts from the fundamental drama of the character and their needs. We may have to put effort into replenishing your store of fresh blood before you can use it.
Presumably if using bagged blood worked just as well as people, everybody (everybody but the degenerates who enjoy hurting others) would just use it, and there would be peace between vamps and humans?
While the idea of blood bags is cool, maybe your source instead provides access to the donor? Maybe they are willingly giving to vamps for some reward? Or maybe they are tricked into believing they are giving to 'medical research' or something (but still willing, so not triggering Corruption, that seems a bit like sophistry?). It can turn into a complicated network if we are not careful, though.
Think about what you want this to do to the story. Remember that you are a predator, a monster, maybe don't take that away with the easy-out of storing up batteries of cold blood bags.