If you don't send the signal from OUT [PRE] eventually back into IN [POST], then the two sections will be completely independent. You'd have to use two exp pedals or or a MIDI controller to adjust the output volume of each.
You'd probably want to do the opposite configuration - acoustic through the PRE section and vocal through POST section. The IN [PRE] is a guitar-conditioned TS jack, whereas the IN [POST] is a line-level TRS jack (+4 dBu or -10 dBv, switchable in settings). You'd need an external mic preamp of course, and it likely has line-level outs, so a perfect fit for the POST section.