Sounds to me like a case of Dog Stomach Flu: I don't know if there really is such, but ours gets periods of stomach ills which I call the flu. Her actions/reactions remind me of our seasonal stomach flu, thus that is what I call it.
Around here we treat with rice mixed with bullion flavoring - they come in cubes and in packets and in jars. One cube to a cup of hot water, mix until dissolved - one teaspoon of the loose stuff to a cup of water. Cook the rice in the 'soup' water, let cool. Chicken flavoring appears to be slightly easier on the stomach and even a sick dog will eat a little.
There are pet grasses sold through pet stores, its wheat grass. You can either buy the already grown grass (cats especially love it) or you can get wheat grass seed and plant a tray:
http://www.sproutpeople.com/seed/wheatgrass.htmlBTW Wheatgrass is good for humans too

Let the dog eat grass, that will cause the dog to vomit - this is a good thing since the dog is purging the stomach.
Dogs are prone to develop wheat sensitivity as they get older, not to the grass, but to the wheat seed/grain that we make flour out of. Actually its a gluten sensitivity, part of the make-up of wheat grain. It can cause skin conditions, upset stomach, diarrhea, vomiting, and a few other not so nice things. Onset of an allergy can be sudden, while the dog may have eaten wheat for the past 9 years, now suddenly its sensitive to the wheat.
Dog food is bulked up with wheat (flour), thus you need to get one that is not. Rice and lamb diet food (expensive? yeah sort of) has no wheat in it.
It is possible that the dog food you have is causing the problem. Switch to a lamb and rice diet and see what happens - its a bit easier on the stomach and good for the 'dog stomach flu' for a week after the illness - see if that makes the dog feel better. Once well switch back to whatever you used before, if symptoms return, take the dog to the vet and say what you have done.
Around here we always keep a bowl of dry kibble down. Each new dog that comes in our house goes through a week or two of scarf until they vomit - but in the end they understand that the food is always there and will change their eating patterns. Our current dog will take a mouthful of kibble to her blanket (bed) and drop it, then lay there and eat a piece of kibble, snooze a little, take another kibble.
Her over all consumption of dog kibble is less than 2 cups a day - she is a lab/rott mix - a fairly large breed - wider and more bulked up than a black-lab due to the rott.
If the dog is scarfing down its one meal a day, try leaving food down all the time - yes for a week or so the dog will go crazy, however once it gets the idea that food is always there it will 'graze' and not be a glutton. This will have a long term positive impact on the stomach. Dog kibble swells in the stomach (Just add water and see) unfortunately a dog can make themselves sick or sicker by eating dry kibble to fill an empty stomach, the kibble swells and the dog vomits to purge the excess, gets hungry all over again, scarfs again - sometimes even eating what it has purged.
Expiration dates on vitamins, drugs, etc usually mean that the medication is no longer at optimal potency. While you are dosing the dog if you give the dog the expired one, it will not be a full dose, it will, after a month post due, be slightly less potent, and mayn't do the trick.
6 months after most drugs are so low in potency you are better off taking none. In some cases the chemicals decay into toxins - so you really do not want to give anyone those expired drugs too far past expiration date.