MID400 used in "Unsaturated" Mode
Your spec is an RMS Vac voltage threshold which must use a precision Vref and some time duration to trigger a logic level for Brownout. Using an optoisolator  by increasing Rin is not a precise method because of wide gain tolerances.

There are many ways to sense Vac {rms,pk,pk-pk,avg} and the easiest is avg. using a conversion from rms to avg. for choosing Vref. This requires conversion of AC to DC avg then subtract or compare with Vref with some LPF to insert a time delay a.k.a. Group Delay of filter.

Does this help you to find existing solutions? Or do you want to create your own design? Or do you need more hints how to implement each step of conversion?

For example full wave bridge is done by the bidirectional IR input diodes with RC filter for DC average then offset with a large Zener = 90% of  req’d threshold  then choose Rin to Optoisolator and allow diode and R drop for 10% or other more precise methods.  

It all depends on your error tolerance design spec for 180Vac rms.  Start your question with this.  

How can you detect a “brownout” at 180Vac rms for “?” second with ? % error using MID400 IC?

The “Unsaturated range” depends on temperature , input current of IR Vf (1.0~1.2V) vs If and hFE of internal phototransistor ( or Photodiode and Transimpedance Amplifier) near saturation which may have a wide range but not explicitly specified. This is implied by the Turn On and Turn Off values of Vrms vs Rin which you want to be close but are not without a series Zener.  This is why an error tolerance is critical and requires a precise tolerance calculation of Vin - Vzt (Zener or TVS bidirectional threshold)