Arturia have a setting in the lower panel where you can change the VU calibration. Default is -18dBFS
Plugin Alliance depends on who coded the specific plugin as they are all done by different devs. Many have metering though so it’s quite easy to measure. The ones made by Lindell have different options in the settings menu. Several have adjustment controls too but there’s no real consistency with them.
For IK, again it depends on the plugin. The LA2A, Fairchild and 1176’s have metering buttons, so they’re quite easy to just run a sine wave into and see. The VU on tape machines is literally showing you its definition of 0VU, so those are easy too.
The Distressor, SSL Bus compressor and 33609 are easy enough to compare with UAD to verify (-11dBFS seems to be a common value for IK, and it’s what matches UAD closest).
