Wawa:
The ADS1115 and MPX pressure sensors are a poor match. The ADS is absolute and the MPX is ratiometric, which makes the setup dependent on pressure and supply voltage. You must therefore also take supply voltage into the equation, which could be a pain (ripple/noise)
Absolutely correct. Luckily for the OP the solution is not that hard: connect the sensor's Vcc supply to a separate input of the ADS1115 to measure the actual voltage the sensor is powered at. Ripple should be taken care of by proper decoupling as suggested in the datasheet.
It seems that there's no 3.3V version of this sensor available (I've used the MP3V5004 before, works great for water level measurement) or that would offer another solution: use the 3.3V output of the Arduino. That's about as stable as can be.