We all feel your pain! A lot of kids have taste issues, a lot have texture issues, a lot have problems just sitting down for a meal. Once they get in the habit of eating only a specific thing, it's really hard to break that. She seems to have found the comfort food of choice, the frozen burrito.
You're right that she needs more than a multi-vitamin. In fact, most doctors don't even think the vitamins are so important because they are so difficult to absorb and they contain a whole lot of stuff you don't want your kid eating. The warning labels alone are enough to give parents pause. So most of the time they are a waste of money. No vitamin works in a vacuum, and there are over 70 nutrients (vitamins, minerals, trace elements, etc.) that need to be present in the diet every day for any of them to really work effectively - that's based on research on the workings of healthy cells. "Just eating right" isn't enough either - the AMA released findings well over a decade ago that everyone has to supplement. It seems that a fully balanced supplement, ideally with a patent (proving safety and effectiveness as well as uniqueness), and not in pill form (because of absorption), and at optimal nutrient levels (not the RDIs that are so out of date) works far better.
I think you might look at 4 things: not asking her what she wants anymore, not buying frozen burritos or other processed foods, spending some of your money on a better supplement, and pure deception (hiding nutrition in foods she already tolerates). At 8, she's not going to starve. She'll spit it out and be dramatic, and she'll get you into a panic about her not eating anything, but she will eventually change her eating habits. Start slowly. She's absolutely got to get some protein in at breakfast time - toast and juice just fill her up, and I get that you don't want to send her to school on an empty stomach, but maybe you can start on the weekends or vacation days. If she sits there for a few hours with no breakfast, she'll survive. But I did a great French toast with more protein in it for my son, and what we call "power pancakes" that had all kinds of hidden gems in them without him knowing the difference. I also made them ahead and frozen them, just popping out what he wanted that morning. I can help you with that if you want.
You could spend some time together on the weekend making some meals. You could improve what's in a burrito by making your own and hiding plenty of nutrition in there - over time, you can increase the amount of other ingredients as her palate adjusts. If she likes tacos, I wonder if it's the beef she likes, or if she likes the idea of assembling it herself. If it's the beef, I found I could add a lot of nutrition to a basic hamburger and meatballs, including beans, spinach, wheat germ, etc. You're already got her eating meat loaf, so you can add a whole lot to that recipe. Falafel is another place to hide things. First of all, it's chickpeas, but you can either make your own or start with a mix and then "stretch it" (and reduce the spices/salt) by adding your own ingredients. Maybe putting that in pita or lavash bread would feel like a taco to her? Maybe you could put turkey and cheese on a wrap, and then branch out by adding a few other ingredients. My son started with enchiladas and not much inside, and then we added more and more. A friend of mine has found her kids will eat vegetable soup or chili -they eat the beans just fine, and then she grinds up all the stuff they don't think they like, such as zucchini and tomatoes and other vegetables. I also make super healthy chicken nuggets and oven fries. If your daughter wants spaghetti, you can give it to her but maybe bump up the fiber by using whole grain pasta and then hiding a whole bunch of stuff in the sauce or the meatballs. Let me know if you need recipe ideas. Between these changes and the supplement, we've all been super healthy for years. No worries anymore.
Have you seen any of the cookbooks that disguise food? Putting pureed butternut squash in mac & cheese, for example? That's Jessica Seinfeld's claim to fame, but there are others. Making a cauliflower & egg pizza crust, and then they build their own pizza from there? That was on Rachael Ray the other day.
What you can do is also insist on a certain level of manners and the absence of drama. And if everyone in the family participates in meal prep and meal eating, and that's where all the fun is, as long as she doesn't get a free pass to watch TV, she may join in.