Gastric secretions only contain approx. 15 mmol/L of potassium. Therefore relatively little potassium is lost directly through vomiting.
However colonic losses can amount to 50 mmol/L of potassium in diarrhoea, increased further if the losses are from the distal colon eg villous adenoma of the rectum. This can lead directly to hypokalaemia.
In addition, any excessive loss of fluid, leading to ECF volume contraction will cause renal potassium wasting.