If vomiting occurs involuntarily and without intention, the fast is not invalidated. However, if a person vomits deliberately, the fast is thereby invalidated. In this regard, there is no distinction between vomiting a mouthful amount or less.
The Prophet (peace and blessings be upon him) said:
“Whoever vomits unintentionally does not have to make up the fast. However, whoever induces vomiting deliberately must make up the fast.”
(Abū Dāwūd, Ṣawm, 32; al-Tirmidhī, Ṣawm, 25)