Welcome to you and Lyca. Don't beat yourself up about giving her steak -- very minor.
There are many reasons why Lyca could have soft stools and diarrhea, and there might be different reasons for the two. Maybe concentrate on getting over the diarrhea, and then start investigating the loose stools?
In addition to giving meds, what did your vet say about it? It sounds like s/he thought it might be a bug she picked up. Were there any tests, like for worms (ewww)? Sometimes, even after a dog has been wormed once, they linger on. Or there could be another kind of worm (ewww).
When a dog has diarrhea, I fast the dog for as far up to 24 hours as I can stand it (usually I cave before then), and then I offer bland food as suggested above -- pasta boiled with skinless chicken, or rice cooked with defatted hamburger. Mostly pasta or rice, with just a little meat to get them to eat it. Then I transition back to regular food, as suggested above. Maybe you could stop giving the vitamins and the canned food (probably higher in fat than the kibble?) while she's recovering? My guess is the rice didn't hurt Lyca's sytem. There are some rare rice intolerances, but usually, white rice is given as a bland food to help calm down the intestinal system. (Most of my dogs can't digest brown rice.)
Offer lots of water (or low sodium broth, if they don't want water) when a dog has diarrhea, so they don't get dehydrated.
Now, the loose stools -- How long have you had her? Sometimes it takes a while for dogs' stress levels after a big change to level out, and stress can affect the tummy.
The too-much-food sounds like a possibility. All the kibble she can eat, canned food, steak -- Lyca must think she's in dog food heaven! How much are you giving her compared to the recommendations on the food? Plus, each dog's metabolism is different. I feed my dogs less than the recommended amount and they maintain their weights -- we don't have the most energetic of lifestyles.
Two of my greys have had stomachs that could digest a battleship, and three have had varying degrees of sensitivity. It may take a lot of cautiously trying different foods and add-ins. As long as she seems happy and healthy otherwise, it's probably not too urgent of a problem.
Hoping you and Lyca get some sleep tonight.