VG-99 does work as USB Class compliant Audio when you do not use the "Advanced" USB mode - bit depth is limited to 16 bits - hence the noise in amp sims (Jamup / BIAS FX) , but adequate for Mobius Looper or IOS recording
And this is odd because I could have sworn that prior to Sierra I had a 24-bit option for the VG-99 but last time I checked it was locked at 16 on my iMac. I'm checking into this once I get home (facepalm).
As I recall, you posted success of VG-99 with MacOS Sierra - but did not update to VG-99 USB Driver, which is a very unique situation (all others I know cant do that)
Successful VG-99 USB audio on MacOS Sierra with older VG-99 Core audio driver for older version of OSX might be explained if your VG-99's USB settings.VG-99 "USB Standard Mode" = USB Class Compliant mode
- and will work with Mac, IOS, Android, Linux - as a USB Class Compliant Audio interface, but its restricted to 16 bits.
If you want 24 bit audio over USB, must change your VG-99 USB Settings to "Advanced Mode" , and use the proper Roland VG-99 USB Audio driver for your operating system https://static.roland.com/assets/media/pdf/VG-99_OM.pdf