By default, Vacation, Sick, Holiday, and Compensation time is tracked with predefined activity items: VAC, SICK, HOL, and COMP. These items, combined with hours allowed for each employee, are used to process PTO and time entry reports.
The PTO widget is not showing vacation time because that time was probably entered using a wrong activity item. You can customize these codes to your preference in Settings > Time & Expenses > Personal Time Off and even change their labels in the Custom Labels screen.