Vitamins ain't gonna help anyone from L.A., lololololol!!!!! Just kidding of course. There is a book by a vet named Belfield, entitled How To Have A Heathier Dog. You can get it from Amazon. Well worth it.
In general tho, the two most beneficial vitamins for dogs are C and E (hhmmm, just happens to be my initials too!! How 'bout that!) At least 500mgs per day per dog. If your dog's poop gets mushy, cut back on the amount of C. For E, 200 IU (international units) is the mantainance dose.
For puppies of the larger breeds, 500 mgs twice a day is beneficial. For theraputic dosages of C as many as 35,000mgs a day have been given I.V.
And yes, dogs can take people vitamins. A vitamin is a vitamin is a vitamin. The doses may vary, and dogs do not need supplementation with the same vitamins as people.
OK.......are you confused as heck now, rofl???