My naive solution was going to be to assign the non-internet devices a smaller subnet with a bad gateway, and have all the "bridging" devices have IP addresses

My naive solution was going to be to assign the non-internet devices a smaller subnet with a bad gateway, and have all the "bridging" devices have IP addresses in that subnet, but with the real subnet and gateway. I'm sure VLANs are superior, but if you could tell me what's dumb about my idea I would appreciate it. Meanwhile, do the VLANs allow the "enumerate devices" style multicast to come down from the regular LAN and be responded to?