I breastfeed and supplement now that I'm back at work and my supply is down. I use similac supplementation and it works good for me
I think its better for u to produce more milk and breastfeed if baby doesn't keep formula down, try eating oatmeal every day and of course drink lots of water cause that's usually the main problem with decreased milk production, plus its best for your baby, there's a breastfeeding forum on here with very helpful and supportive mommies, they say fenugreek also helps with milk production and lactation cookies, if u still decide to supplement maybe try different formula or try to give her only half of feeding amount and then burp her and see if she'll throw up smaller amount, maybe she's overfeeding, how much formula r u giving her?