3.3V very low power consumption buffer for sensor output
That chip does not give an analog output, it is a digital multiplexer, your looking for an analog switch and a buffer, or 2 buffers, 

I would suggest picking your favorite distributer and sorting by supply current, narrow it down to what you need, and then cross check the datasheets of the shortlists 

E.g. here is a search link for any op amps under 1uA supply current, It may not remain valid as time goes on, but as it is a single column it should not be hard to reproduce. 

Mouser Op Amp List

Equally if your only measuring that one voltage, you may make life easier for yourself by feeding the buffered VREF signal into the arduino VREF pin through a 5K resistor (with a 10-100nF capacitor to ground at the arduino vref pin) this way once external vref is selected you get results as a proportion of vref, with a lot less math.