There's another thing that I don't get.
Hope I'm not too spammy.

VLANS can be configured at:
https://<m0n0wall>/interfaces_vlan.php

But creating a VLAN doesn't seem to create a new virtual interface.

So how do I tell m0n0wall what networks are behind that VLAN, and
assign the m0n0wall an IP address on that VLAN?

If it's not obvious, I want to use one VLAN per customer and use
m0n0wall as a gateway and firewall.

(Other than assigning an IP address, I'm also confused about how to
define anti-spoofing (so far I guess it's manual) and where to put
VLAN rules (on the ruleset that's specific for the VLANs parent
interface perhaps?))