make sure that there is no milk in the pipe - which can be caused if the milk is too hot, you shake milk with the pipe inside or you reheat milk in cup of hot water etc..if there is milk in the pipe then the air that bb blows out when drinking the milk cannot escape thru the pipe and therefore bb will have a lot of "fung" still inside after bb finishes milk
also is bb drinking milk comfortably? if the nippple is too big, milk will flow too quickly for bb making bb take in more air
these are my suggestions based on my own experience - hope it helps and your bb stops vomiting soon.