The Devil and all his legions are at war against Our Lord, and all his Angels and Saints.
When The Evil takes possession of someone and they seek the aid an exorcist, the proof of possession must be fully ascertained before he can be given permission from the bishop to perform the exorcism. There are many things that must come into play here: doctors in various fields of medicine, etc...to aid the exorcist. Vomiting is just one of the many signs of possession and at times in great volume.
According to the Vatican guidelines issued in 1999, “the person who claims to be possessed must be evaluated by doctors to rule out a mental or physical illness.” Most reported cases do not require an exorcism because twentieth-century Catholic officials regard genuine demonic possession as an extremely rare phenomenon that is easily confounded with natural mental disturbances. Many times a person just needs spiritual or medical help, especially if drugs or other addictions are present. After the need of the person has been determined then the appropriate help will be met. In the circumstance of spiritual help, prayers may be offered, or the laying on of hands or a counseling session may be prescribed. The exorcist might not perform an exorcism if he doesn't know the person.
Signs of demonic invasion vary depending on the type of demon and its purpose, including:
1.Loss or lack of appetite
2.Cutting, scratching, and biting of skin
3.A cold feeling in the room
4.Unnatural bodily postures and change in the person's face and body
5.The possessed losing control of their normal personality and entering into a frenzy or rage, and/or attacking others
6.Change in the person's voice
7.Supernatural physical strength not subject to the person's build or age
8.Speaking or understanding another language which they had never learned before
9.Knowledge of things that are distant or hidden
10.Prediction of future events (sometimes through dreams)
11.Levitation and moving of objects/things
12.Expelling of objects/things
13.Intense hatred and violent reaction toward all religious objects or items
14.Antipathy towards entering a church, speaking Jesus' name or hearing scripture.
It is not hard to imagine that the Devil tries his upmost to remain hidden, but once he has been discovered through an exorcism he will try his hardest not to be expelled. If he has to spit, vomit or worse at the exorcist he will. I had a priest exorcist tell me more than once: "You can not imagine how many times I have been spit in the face!"
Exorcists are required to be very prudent when asking questions about how a person became possessed:
15 But necessary questions, are for example; about the number and name of the spirits inhabiting the patient, about the time when they entered into him, the cause thereof, and the like. - De Exorcizandis Obsessis a Daemonio
Fr. Gabiele Amorth in his book An Exorcist Tells His Story (Pages 137 -139) has this to add to questioning the Demon, vomit and liberation:
Norm number 20 of the Ritual suggests asking the Demon the cause of his presence, particularly whether it is due to something the individual ate or drank. If this is the case, the exorcist must command the person to vomit. If instead the curse is due to some foreign object, the exorcist must ask to be shown where the object is hidden, find it and burn it.