I took this guy from here - http://rlx.sk/en/accelerometer-gyro/3582-vibration-sensor-module-801s-er-sen801svs-.html and without noticing I have applied 3V3 instead of the advertised 5V to it. It also seemed to me it has worked fine. Can anyone tell me why it has still worked or if it makes any difference in this case ?
The module contains an extremely simple circuit:
The sensor is probably a mechanical one (metal ball or spring) so that works at almost any voltage. The opamp is an LM393 which works form 2 V supply and more.
So there's no reason why it would not work at 3.3 V.
Note however that I personally would not use this circuit as it has issues in its design. There's a reference circuit in the datasheet of the RZ801 which will work much better.