I purchased the Zoom Livetrak L-8 and, after installing the driver, VMIX independently recognizes each of the Livetrak's audio inputs. The problem I have is when I raise or lower the faders it does not raise or lower the volume in VMIX. When I press the Mute key on the Livetrak, the sound in VMIX is not muted either. It only raises or lowers the volume if I press the Gain keys on the Livetrak.
I have been told that it may be because the Livetrak L-8 is not an audio interface but rather a video mixer.

Has anyone been able to get the Zoom Livetrak L-8 working correctly in VMIX?

Is there an audio interface that is fully compatible with VMIX, that separates the channels in VMIX, records the different channels on cards, etc.?
I have the Behringer Euphoria 404HD and it works perfectly but it does not record to an SD card.

To add onto this, most ASIO devices--or at least, most consumer ASIO devices--are pre-fader to cut down on processing.

To the extent that this is "a problem", it's a problem that you're going to have with pretty much everything in this price range.

vMix provides digital faders, as well as the option to add digital gain, to allow you to manipulate audio however you need to.

That should eliminate any need for faders on the interface itself to control the volume going to vMix.