You can use In-Town Check-In at Hong Kong or Kowloon Airport Express stations anytime from early morning on your day of departure, where you can check-in your luggage and receive your boarding pass. If you have plenty of time as you say you do, why not enjoy Hong Kong? That's an awfully long time to spend in any lounge, however good it is. On my last VS flight from HKG I decided to enjoy the city rather than spend hours in the Clubhouse. Just head back to the airport later in the evening, the train takes only 25 mins from Kowloon. The lounge is not worth missing half a day in Hong Kong!