Even though my 3 yr. old male cat is being fed 4 times a day (last, around 9:30PM), he still vomits once during the night (copious amount of clear liquid). He has recently had a splenectomy with the removal of 10 mast cell tumours from under his skin. Otherwise, he has never been perkier or more interested in his food. Why is he vomiting at night?

It could very well be a hairball he's having trouble passing. Either give him hairball medicine, or a teaspoon of butter, once a day for a few days and see if it helps.