My bub is 3 and a half months old and she gets ...

My bub is 3 and a half months old and she gets mild reflux. i want to slowly introduce her to formula one or two feeds a week and i tried her on s26 gold whey dominant as the book says that is the closest to breastmilk. she tried it once and vomited it up. should i perservere with the s26 gold or try the s26 thickened formula for babies with reflux? plus is it ok to do what i am doing? slowly introducing a formula feed to her once or twice a week. i am still breastfeeding for as long as possible.