I have had that exact issue. Most of the assemblies will have adhesive to keep that little stick (usually green or red) inside the button, however it is pretty much useless due to the amount of pressure needed to actually flick the switch, in turn dislodging the button.
Fix - Super glue. I know it sounds dodgy but it does the job. Just be very careful not to glue any of the circuitry or you will run the chance of ruining the entire assembly. Below are two pictures, the first shows where is safe to glue displayed by a circle. The arrow is the vibrator circuit.
Place a decent amount of super glue in the circled area but make sure to not use to much as it will cause seepage and it will possible leak onto other parts of the circuit.
Once the glue has been applied quickly place the vibrator circuit on top holding down firmly for at least 5 minutes. Remember, the longer you hold it the better.
Once this has been completed you will find that the red stick with stay firmly in place with the button as long as all screws are replaced in their correct holes.
Hope this helps!