outdoor cat and litter box

Question
My two year old female cat is both outdoor and indoor. She spends nights indoor because I noticed when she stays out at night she comes in, in the morning and vomits. Throughout the day she is in and out of the house, to say hi, to eat, and TO USE THE LITTER BOX. Why and is there a way I can get her to go outside; if I even should.

Answer
Cats prefer to do their business where they feel safe and undisturbed, and in her own private house is safer than the great outdoors.  

You could try eliminating the litter box and see if that works.  However, monitoring your cat's "output" is one of the best ways to monitor her health.  If she goes outside, you'll never see signs of diarrhea, parasites in the stool, blood in the urine, frequency of urination, straining in the litter box, or lack of bowel movements (indicating constipation or perhaps obstruction).  So I would personally recommend you keep the litter box.