I'm sorry to "necro" this old question but I was having the same issue and resolved it myself. I found this question when I was searching for help and figured it might be useful to someone else if I responded.
Even though I enabled DHCP on the only NIC on the server in the wizard it needed to be re-enabled in the DHCP management console after I completed the installation of the DHCP role. I, like the OP, also used WireShark and confirmed the DHCP Discovers were being issued and seen by the server, but the DHCP Service had not been enabled on the NIC by the wizard and so did not hear the Discovers.
Woah, there. What you're saying contradicts itself. You say "single subnet" in one point, but then "VLAN each site" in the second point. Then you say "the networks will NOT be routed". Are you sure you know what you're saying here?
Typically 802.1q VLANs are deployed in a one-to-one relationship with IP subnets. Each 802.1q VLAN acts as an independent Ethernet broadcast domain and, as such, broadcasts from one VLAN (like, say, a machine ARP'ing for another machine in the local subnet) won't be forwarded between the VLANs. Splitting a single IP subnet across multiple VLANs requires a "smart" bridge that can do proxy ARP.
How are you planning to get ARP to work between these various VLANs?
If you really want to eliminate "cross-site 'chatter'" then what you really want is a subnet for each physical location, a router at each location connected to the "MAN" to route traffic to the other locations, and "ip-helper" functionality in each router to forward DHCP requests from the various locations to the central DHCP server.
What it sounds like you don't want is a single big subnet with a bunch of bridges running proxy ARP, in my opinion. Your DHCP inquiry really, really speaks to an underlying desire (though you don't know it) to have per-location subnets with DHCP scopes for each.
To speak to your question specifically re: DHCP: A DHCP "scope" is a range of IP addresses and options that a DHCP server will "hand out". The DHCP server chooses the scope to choose an address based on either the network interface the request is received from (if it's a broadcast request) or the address of the DHCP relay agent (if it's a relayed request).
Some background: Best way to segment traffic, VLAN or subnet?
Best Answer
The most common way to do this is to split your IP range between the 2 DHCP servers and have both running at the same time. Clients will then recieve an address lease from whichever server responds first. In the event of the bridge failing, those with leases from the server on their own subnet will be fine, any with leases from the other subnet will get a new lease from the other server when they renew.