Hi, sorry I didn't have a chance to look at this yet :S
There seems to be a similar problem with the dbusmenu (at least in Ubuntu), where the menu is displayed incorrectly.
I'm inclined to think that this happens because our Vim menu is placed inside a QToolBar (to make it floatable). Since this menubar is not the "real" menubar for the window it is hard to access it by normal means.
Next steps to handle this:
Use the default QMenuBar from the main window and check if these issues disapear
If (1) is true consider removing the floatable menubar completely.