It doesn't sound to me like the cat has serious diarrhea. Feed the cat nothing but hard food ever! My Vet told me to NEVER feed my cats canned food. He said hard food like Purina One "Sensitive Systems" is great. I give my cats nothing but that and it cut the vomiting down to almost nothing. They never get loose bowels either.
Some cat food has enough vitamins in them that you don't need to give them any other supplements. That is what I have found. Give it a try.