He probably vomits cos it is too rich for him. 3/4 pouches a day is quite high - although it is the manufacturers recommended amount. Each pouch is 100g, so 4 is equivalent to one tin of food (again, what they recommend, but I struggle to get one cat to eat a full tin in one day). You can feed a completely wet diet with no harm, it is complete food, and at teh end of the day, the high meat content foods are relatively new, all the cats in their teens/late teens would have been fed Whiskas and the like to start with.