Nice looking opaline sky white and light green cocks. Sadly your description of your birds being seed junkies is quite typical of pet store budgies. It is easier to convert 2 or more birds over to a healthy diet "more quickly" than one bird. I put that in quotes as a relative term. A good healthy diet should consist of fresh veggies, fruits, pellets and only 20% to 30% of the diet made up of seed. Once your birds are on a heathy diet they will no longer need vitamins. I will caution you about putting "stuff" in their water source is a good way to breed bacteria. If this is the rout you choose to take make sure to throughly wash out the water cup was soap and water DAILY.
To work on converting them to a healthy diet start with something easy, the greens. Make a game of it, budgies by nature LOVE to roll on the wet grass to bath. Tie a bundle of parsley and hang it from the cage top so it is about beak level the their favorite perch. Make sure you rinse it throughly and leave the bundle drippy wet when you hang it. Once they get use to the new object that you hang fresh daily in their cage they will start to play with it. Once they are eating it regularly you can move it loose to the bottom of the cage. Next you might add a bit of chopped mustard greens or turnip greens or kale. Once they are eating all of those try adding a bit of grated carrots. What you will ultimately be doing in the end is making a tossed chopped salad for them. To this mixture you can add chopped broccoli, cabbage red or green, carrots, beats, sweet potatoes, apples, etc.
Foods that are toxic to budgies are: alcohol, chocolate, caffeine, rhubarb, potato eyes, tapioca, avocado, potato chips, fries etc. Budgies should not be drinking milk out of your cereale bowl. However if budgies have been on antibiotics for being sick giving them some plain yogurt is good to replace to good gut flora that was killed by the antibiotics. Don't feed your birds frozen yogurt from a store dispenser. You don't know how well the parts were cleaned and many birds have become sick by eating this contaminated yogurt. It may not be enough to make humans sick, but birds have a more delicate system.
If your local pet shop or local book store doesn't have a good selection of budgie books you can find them on
http://www.abebooks.com I have order birds as far away as England and Ireland without any difficulty. Even with shipping the prices have been better. There are several ways to search for books which is quite handy.
If you are looking for a more in-depth book I would suggest Cult of the Budgerigar. There are 5 editions of this book, all collectors items for those collecting budgie books. Edition 1-4 are the best. All of these book have long since been out of print, but the information is sound especially if you decided to take your interest in the budgie hobby further into breeding.
Since you are new to birds I would also suggest you take a subscription for Bird Talk Magazine. It is a good way to learn the basic of bird keeping and about different species of pet birds. Another thing that is highly recommended it join your local bird club. This way you keep up with local events, find out where the best avian vets are and have close by help if you need it.
Hope this helps.
Kerry