Hallo! If the hardware is Riffle version 0.1.8, the relevant schematic is here -- and, so far as I can tell, VBAT should simply be the battery input voltage. But I'm also very hesitant to bet against diagnostic performed by the generally-very-thorough @cfastie, so I suspect I'm missing something.
In any case: getting a Riffle (which does indeed operate at 3.3V) to talk to 5V sensors will require two things vis-a-vis power: a) you'll need to find a way to provide 5V to power the sensor, and b) you'll need to find a way to talk to the sensor with compatible signal levels. If, e.g., the 5V sensor outputs an analog signal that ranges from 0..5V, the Riffle's analog inputs can only handle 0..3.3V, so you'll want to use a voltage-divider (easy way) or a 'level-shifter' (a special chip that lets different voltage levels speak to one another). Another examples: if e.g., the 5V sensor uses the i2C protocol, there's the possibility that the Riffle's digital 3.3V signal HIGH values (3.3V) aren't high enough to register as a 'HIGH' for the i2c sensor chip in question -- a case that might also be handled with a level-shifter, I think.
For a nice general discussion of getting 3.3V hardware to talk to 5V hardware, see this post by Hackaday.