I did this modification and the program now run only at ~70fps wicht suggest it is doing the VSync.
But, now I have one more question (sorry for abusing your patience, hehe).

If the double buffer now is managed by the gdi, there is no need to keep my own backbuffer, ok?
I could use something like this pseudocode without trouble?

Code:
PROCEDURE DRAW_EVERYTHING
BEGIN
  DRAW_ALL_OBJECTS_TO_CANVAS;
  SWAPBUFFERS;
END
Thanks again!