Find out how and in which sequence the relevant functions (LockPhysicalScreen, Enable/DisableRefreshPhysicalScreen, RefreshPhysicalScreen) are used in the firmware, and try to repeat it in your code for vid_bitmap_refresh().
For instance, in the very recent S90 port I see the sequence
ScreenLock();
EnableRefreshPhysicalScreen();
ScreenUnlock();
which is probably there for a good reason (as far as I understand, ScreenLock=LockPhysicalScreen and ScreenUnlock=RefreshPhysicalScreen).
As to finding the correct addresses, if these functions are named in the firmware, as they used to be, it shouldn't be difficult to identify them beyond doubt.