Can't force it down, because it's too big, and she's too small.
This is the key. It sounds like the vitamins are simply too big, and your dog finds chewing them to be unpalatable. You don't say whether or not they're chewables, but even if they are some dogs simply don't like the taste or texture of certain items.
You have three basic options:
- Get a different brand, preferably in a smaller size and in a flavor your dog likes better.
- Use a pill crusher, and mix the crushed pill in with something strong enough to overcome the gritty texture and unpalatable flavor of the vitamin. I've had good success with canned green tripe, but your mileage may vary.
- Use a pill splitter and some small Pill Pockets to give her pill portions that she can swallow whole. If you don't want to use commercial pill pockets, wrapping a split pill in cheese or meat might work, too, but the pieces have to be small enough to avoid chewing and releasing the taste of the pill.
As an alternative to vitamins, you might also consider a palatable nutritional supplement like Wysong Add-Life. I've certainly met dogs that don't like the powdered texture of the Add-Life, but most have found it delicious, especially when mixed with a little water or chicken broth. Your mileage may vary, of course.