I don't know exactly why you are giving your dog extra vitamin B, but assuming you have good reason, you should stick with well known brands. Vitamin B is water soluble, meaning excess is eliminated through the urine rather than being stored in the fat (like vitamin E).
If you give your dog extreme amounts, you could run into trouble. Stick with a brand name vitamin B complex of 100mg--one per day. For a dog the size of a St Bernard, it should be fine. I take 100mg / day and am 105 pounds.
I have given my Dalmatians (45 and 55 pounds) 100mg./day to for heart and joint support (they are 14 and 15 years old).
If you buy the capsules, you can open them and sprinkle the vitamin B on your dog's food. If you buy the tablet, you can coat it in peanut butter and feed it to him. (vitamin B tastes really bad).
Hope that helps.
Here is an online vitamin site where you can buy discount vitamins...
If a vet did not tell you to give the dog this vitamin, then you should call him and discuss it firs. I am assuming that he did not since you do not know the dose or the best brand to give the dog. Essentially, the brand is probably not that important. It is important to know how much and what for are you giving the dog the vitamins for.