I am assuming the netbsd box is a dhcp server, and something else is sending requests. Run tcpdump on wm0 and the vlans, and see what those requests look like. I think dhcpd is using bpf, but as tls points out it shouldn't be auto-unwrapping the vlan tags. You haven't established that you aren't getting untagged requests and requests on a vlan at the same time.
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