Prior to vomiting, Montag and the other firemen had just invaded the home of an old woman who was hoarding books. Montag, against his better judgment, steals one in the process, so he's already feeling adrenalized and sickened by his actions. When they proceed to douse the woman's home in kerosene and prepare to burn it down, she produces the matches to do the act herself, encourages the firemen to leave, and then sets herself and the home on fire. This sickens Montag, as well as most of the other firemen, who aren't accustomed to this sort of wanton "disregard" for one's own life, particularly in favor of books.
Later, as Montag is going through his usual routine of trying to talk to Mildred while she occupies herself with her parlor screens, Montag vomits when he catches a whiff of the kerosene that's still on his hands. While it's objectively the kerosene that makes him vomit, it's really the memories and guilt that the smell triggers.