Giving her treats because she's not eating her food isn't a good idea. I know you want to help her, but treats are exactly that, treats, for giving them occasionally. Like sweets for people. If her illness is food related, the treats could make her worse. Rawhide treats shouldn't be given every day, but as long as you give her a substantial one based on the size of her mouth, that will take her a long time to eat, they're fine in moderation. It's best not to give her things like that until you find out the cause of her problem though as they could make it worse. If you fill her up on treats then she won't want to eat her regular food as she won't be hungry.
Do you know what bloodtests the vet did? I know sometimes they don't tell you or are unclear, but it'd really help to know what they did if you know? A food allergy test is a bloodtest, so it'd be useful to know if she's been tested for allergies. I don't know about over there, but in the Uhe sethese tests are expensive. When she vomits, what is the contents of it? Is it like regurgitated treats or watery yellow bile? Dogs will vomit bile when they are very hungry. Changing a dog's food suddenly can upset their stomach. It needs to be done gradually to allow their body to adjust. Because they get fed one type of dry food and or dog meat, their body gets used to digesting that one food. If you change the food suddenly to something different, it throws everything out of balance and can give them awful sickness and diarrhoea.
I know you said that your parents don't feed her 'people food' but it really is worth trying to give her something really bland if she hasn't eaten for a while. Boiled chicken breast cut up with some plain boiled rice is good when they're off their food. Or plain scrambled egg with boiled rice (no butter or anything). If she's hungry, she should eat this. But if she does, and she keeps it down, you should feed her this and nothing else until she's better. It doesn't contain as many vitamins as dog food, but it'll settle her stomach, and allow her to build up a bit of strength. If this works, you can then slowly wean her onto her original dog food over a couple of weeks, so as not to upset her stomach. What was her diet originally? Has she ever had problems like this before? Also, what was her feeding routine? Did you put her food down and then take it away after 15 mins or so, or did you leave it down for her all day?
I know it can be difficult when your parents make the decisions about your pets. Try to talk to them about getting her spayed. If she were spayed, she wouldn't need to wear a diaper, wouldn't make a mess, and you wouldn't need to worry about her being in the yard because male dogs wouldn't be interested in her like they are when she's on heat. If this started when she came on heat, then this could be the problem, and she may be perfectly fine once her season is over. If so, then getting her spayed is the best thing to do, you don't want this happening every time she's on heat.
You say she has company all day at home while you're at school/work, and that she gets walked regularly, so it doesn't sound like withdrawal symptoms. If it were, she should eat when you're with her. If this is the first time this has happened it makes withdrawal less likely too.
If she's unsociable around other dogs, a training or socialising class would be a huge benefit as already mentioned.
Has anyone used any chemicals in the yard? Rat poison or anything like that? Or in the house? Is there anything harmful that she could've got to? If she's still active as you say then this is less likely to be the cause, but you need to cover all options.
I really hope you find the problem and that she gets better soon, it's horrible to see your pets ill like that