Answered by Shaykh Amjad Rasheed
Does vomiting invalidate the fast?
If the person fasting deliberately vomits and he is aware that it is unlawful for him to do so, then his fast is invalidated and he has to make it up.
However, if he vomits involuntarily then it does not break the fast. The Prophet (Allah bless him and give him peace) said in a rigorously authenticated hadith: Whomever vomiting overcomes him (r. while fasting), then it is not obligatory on him to make it up; and who deliberately vomits then he has to make it up.
Likewise, if he forgot he was fasting, or was ignorant of its unlawfulness whilefasting, and he deliberately vomits, then it does not invalidate the fast; and he is excused for his ignorance, as is the case with a new Muslim, or one who is living far from Muslim scholars, as it says in the Tuhfa [of Ibn Hajar, a commentary on Nawawis Minhaj] (3: 389).
Amjad Rasheed
[Translated by Sr. Ruqayyah Ahdab]