FYI, here is the relevant text from the Marriott website on room upgrades:

"When you check in, we'll upgrade your room whenever we can at no additional charge. Suites not included. Not available at Marriott Vacation Club International or Horizons by Marriott Vacation Club."

Though I do agree with Michigan Czar, it can't hurt to ask. Just don't expect an upgrade. The thing to remember here is that the vacation club rooms are not really owned by Marriott. My guess is your chances would be based on how sold out the facility is (i.e. does Marriott still control a large block of rooms) and how many of the owners have sold back the room to Marriott (i.e. to receive points or rental money.)