Amp remote line goes high when the VFD display is on, goes low when the VFD display is off.

Telling the difference between full-power-off and Standby is a different animal. Not sure if you need that or not?

Edit: reading your post more closely you say you can't use the amp remote line. So I assume that what you're asking for is a way to tell the difference between full-off and stanby. Sorry.