On the AudioBox 1818VSL, the headphone outputs share playback streams with Outputs 7 & 8. If you want to create a monitor mix for your headphone output in VSL, you must create a monitor mix for Outputs 7 & 8. Additionally, if you want to hear playback from your DAW or computer through your headphones when not using VSL, you must select Outputs 7 & 8 as your main outputs, rather than Outputs 1 & 2.
NOTE: If you use StudioOne 2.0.5+, and you have the AudioBox driver setup as your audio device, using the template in Studio One for the AudioBox 1818 VSL will automatically set up a cue mix for your sub output 7 & 8. Furthermore if you are using the current driver, 1.2+ for the AudioBox 1818 VSL, the VSL should set the routing up for you automatically on all your channels and send aux send output to 7 & 8.
A video explaining the set up for head phone output using the VSL software:
PreSonus 101: Headphone Routing on the AudioBox 1818VSL