You shouldn't get food wet and then leave it.

The big question is: why do they need these vitamins? Was this something your avian vet said they needed? Was bloodwork done to confirm? When was their last yearly wellness exam?

When a parrot is on a good diet consisting of plenty of fresh veggies, a good staple pellet (Harrison's, Roudybush, ect), they don't typically need extra vitamins added to their water unless they have a condition that necessitates it. For example, an egg laying bird should have a calcium supplement. Otherwise, only your vet could tell you if your bird needed vitamins.

If a pet store employee told you that you needed them, that's what a liar looks like. Ask them what the proventriculus is and they'll likely stare at you like a dead goldfish. They don't know very much about the pets they're selling things for.

If your bird is getting vitamins because they're on an all dry seed diet... consider altering their diet. There aren't enough vitamin drops in the world that can make a dry seed diet suddenly healthy. And dropping the liquid vitamin on the dry seed will just make them rot faster. Not to mention the vitamins will largely be in the hulls which the bird doesn't eat. So, yeah. Not very helpful for you.

So basically, vitamin drops for water go in water. And you probably don't need them if you're feeding well.