This is because of VR.employee_id=156
you are not allowing null row.
You can do that :
SELECT VM.vacation_id,
VM.vacation_desc,
isnull(sum(VR.total_hours_applied),0) AS totalCount
FROM EMPTYPE_VACATIONCONFIG VC
LEFT JOIN HR_Vacation_Master VM ON VC.VACATIONID=VM.vacation_id
LEFT JOIN HR_Employee_Vacation_Request VR
ON VR.vacation_id=VM.vacation_id AND VR.employee_id=156
WHERE VC.BRANCHID=20
GROUP BY VM.vacation_desc,
VM.vacation_id
Leave me a comment if this not works, I have some other ideas.