i have the same problem. my boy spit up milk very easy even he's now 13 mths. I guess yr baby doesnt want milk because she feels uncomfortable after spitting, so she just refuse to drink.
If you really want her to drink more, try mix it with oatmeal, cereal, banana. in my case, i feed my boy 6oz in the morning and then i will add another 1 or 2 oz to oatmeal 2 hrs later as snack. you may also mash a banana and then add 1 or 2 oz milk to it. plus the 12 oz she's willing to drink and these extra 2-4oz, you should feel better because at least she can have more milk. if you were to blend the milk with oatmeal and banana, u can feed her with spoon so it may reduce the risk of vomiting.
my boy is a small size baby, vomits a lot since birth til now 13mths, each meal takes more than an hour to finish. he's hard to take care but we need to be patient and add oil! Pls dont say anything like u regret or wanna dump her. add oil!!!