The majority of scholars are of the view that vomit is impure (najis).
It says in al-Mawsoo’ah al-Fiqhiyyah (34/87):
There are different views as to whether vomit is pure or impure. The Hanafis and Hanbalis say that it is impure (najaasah), and both of them based their arguments on different evidence. This is the view of the Maalikis with regard to food that has been changed (by the digestive process), even though it has no characteristics of stools. They quoted as evidence the words of the Prophet (peace and blessings of Allaah be upon him): “O ‘Ammaar, garments should be washed in five cases: stools, urine, vomit, blood and semen.” End quote.
They also quoted as evidence for its being impure the fact that it may be likened to stools, because it is foul-smelling and off-putting.
Some of the scholars, such as Ibn Hazm and al-Shawkaani, are of the view that it is pure (taahir), because the basic principle concerning things is that they are pure, and there is no saheeh evidence that it is impure.
They responded to the hadeeth of ‘Ammaar by noting that it is da’eef (weak).
Al-Nawawi said in al-Majmoo’ (2/549): This hadeeth of ‘Ammaar was narrated by Abu Ya’la al-Mawsili in his Musnad and by al-Daaraqutni and al-Bayhaqi. Al-Bayhaqi said: it is a false hadeeth with no basis. Al-Daraaqutni and al-Bayhaqi stated that it is da’eef (weak). End quote.
It was also classed as da’eef by al-Haafiz ibn Hajar in al-Talkhees al-Habeer, 1/33.
Shaykh al-Albaani said in Tamaam al-Minnah (p. 53), commenting on the view of the author of Fiqh al-Sunnah that vomit is impure:
I say: The author does not mention the evidence for that, apart from saying, “There is consensus that it is impure.” This is a flawed claim, because Ibn Hazm did not have that view when he stated clearly that the vomit of the Muslim is pure. This is the view of Imam al-Shawkaani in al-Darar al-Bahiyyah, and of Siddeeq Khaan in his commentary on it; they do not mention human vomit as being among the things that are impure in general, and this is correct. Then they state that there is some scholarly dispute as to whether it is impure, and they stated that it is more likely to be pure, as they said: The basic principle is that things are pure, and they cannot be deemed otherwise unless there is sound evidence to that effect. Something similar was also stated by al-Shawkaani in al-Sayl al-Jaraar. End quote.
Shaykh Sulaymaan al-‘Alwaan was asked about whether vomit is impure and he said: The correct view is that it is pure (taahir) in all cases (i.e., whether the food is changed or not). The fact that it is off-putting or smells bad does not mean that it is impure.
The basic principle with regard to this matter is that all things are pure unless there is proven evidence that they are impure. There is no evidence to prove that vomit is impure, so it is pure. End quote.
See: al-Majmoo’, 2/570; al-Mughni, 1/485
With regard to whether wudoo’ is invalidated by vomiting, the correct scholarly view is that vomiting does not invalidate wudoo’. See the answer to question no. 2570.