On Authentication Services version 188.8.131.5254 the default_realm configuration is removed from vas.conf when running 'info toconf'.
This line is removing it, it's also the easiest way to see the behavior:
/opt/quest/bin/vastool -u host/ info toconf /etc/opt/quest/vas/vas.conf
Product Defect ID #799583 has been raised to investigate this issue.
At the time of this writing, April 8th, 2019 there is no ETA on when this will be fixed.