I've given this to many animals countless times and it has worked for me, some people just cannot afford to go to the vet, also vets don't always work I know from personal experience.
In fact I've saved lives before by giving them a mix of vitamins, vitamins are natural it would take a ton to even remotely come close to toxic levels, in fact I myself take mega doses daily.
If a person cannot afford vet care for their animals, they should probably reconsider things like breeding. Sure, people fall on hard times and there is aid out there for that (care credit, patient assistance funds, etc) but I urge people to rethink owning pets if struggling to pay for proper care in emergencies is consistently a problem. Not to mention the number of things that can go wrong during animal parturition (the birthing process).
Your information about vitamins is just not correct. While it can take quite a bit to overdose on a water soluble vitamin, vitamins A, D, E, and K are fat soluble and can be very easily overdosed in healthy animals. Considering vitamin D's intrinsic role in calcium homeostasis, it is not something I would suggest using in a puppy (or any animal) before having blood checked. This also includes humans!