difference between vestibule and orifice
The vestibule of the vagina is the space between the labia minora. It has the openings of the urethra, vagina, and ducts of the greater vestibular glands. The external urethral orifice is posterior to the clitoris.
This is a very old method to measure flow rates. Orifice plate is similar in the sense that it make use of a plate with a small hole in between to place restriction in the passage of the fluid. The pressure difference before and after the orifice tells the rate of flow of the fluid.