I'm no authority and my advice is free, and in this case you get what you pay for, possibly less!
But... I believe that, in general, you don't need to provide vitamins to your snakes. Some texts I've read recommend very limited use of 'reptile specific' vitamins that you can get at most pet stores. But there's usually not a recommended dosage (too many variables). Just make sure that the vitamin contains ...er... D3 (please do a bit of research... I just said that out of memory and my memory is terrible!) which is needed for absorption of calcium. Personally, I would use the 'less is more' rule of thumb and at most, dust a mouse's butt with just a pinch and not more than 1 meal a month.
A better way, I believe, to make sure your snakes get good nutrition is to make sure you feed it very healthy mice. If you raise your own, you can give your mice high quality food, maybe a supplement now and then, and so you are feeding your snakes furry vitamins with every meal. The Mouse Factory as a very good reputation for providing good sized, healthy mice (except for being dead and frozen, of course!!).
The lost tail tip is unfortunate, but not uncommon. Often that is caused by that last bit of old skin not coming off for multiple sheds. You said it is 'breaking' off. Watch it closely and if there is any amount of rawness or otherwise unhealthy look to it, see a vet, or at the very least, apply a bit of triple antibiotic ointment to it (like Neosporin) to make sure it doesn't get infected and lose more.
Good luck!