Depends exactly what vaccu-flush unit you have.....
In my 2006 300da, the vaccu-flush unit has 4 duckbills that are a larger size (possibly 2 inch?) on the vacuum generator, and 2 duckbills of a smaller size (possibly 1.5 inch) on the macerator/discharge pump.
Somewhere on your unit should be a plaque or sticker with a list of parts for rebuild.
Edit - auto correct keeps changing vaccu flush to vaccine flush....