In Apple’s El Capitan OS X system, Apple has removed previously available Kernel Mixing functions that have been available up until Mac OS X 10.10.5 which renders the VSL Software inoperable.
If AudioBox VSL 1.3 or older versions of this software and drivers are installed, the AudioBox 22VSL, 44VSL or 1818VSL won’t show up in Sound Preferences, or in Audio MIDI Setup. However it will still show up in your System Report on the USB bus.
In order to make the AudioBox VSL interface work, we need to remove the VSL software so that the interface can work with Apple’s class USB Audio Driver.
Follow this tutorial on how to remove the AudioBox VSL software from your OS X system.
For a more illustrated process, follow this tutorial for a pictorial walkthrough on how to remove the AudioBox VSL software from your OS X system.