Logic level translations
As discussed in the remarks, you can operate the IC with Vcca = Vccy = 3.3V.
But make sure your circuit satisfies the description given in POWER SUPPLIES on page 17 of the datasheet.
So, power VCCY first and then VCCA, and decouple the VCCA pin and VCCY pin to GND as close as possible to the device.  

I'd also advise to get rid of the relay and replace it by (high side) mosfets. This enables you to enable Vout (whether it is 1.8V or 3.3V) after 3.3V is powered up. You could e.g. implement a supervisor IC to enable Vout.