What I would do, personally, is have your "ValidationOfControl" return a value that is used by both BeforeUpdate AND your CloseButton.
For BeforeUpdate you wind up with something along the lines of
IF ValidationOfControl() = FALSE THEN Cancel = True
For your button you get
IF ValidationOfControl() = TRUE THEN DoCmd.Close
In the first, if your validation does not return TRUE then you know to cancel. In the second, if your validation does return TRUE then you know to close the form.