The original topic "ReadyNAS 2304 fails to remove VLAN" by @sully8391 is closed, but I had the same problem and found a solution that did NOT involve reinstalling the OS as @StephenB suggested, and wanted to share that.
To recap, once creating a VLAN, the system won't let you delete it. It reports an error "Failed to remove VLAN" with code 4004020001. I am on a ReadyNAS 2312 running OS 6.10.4 Hotfix 1.
I had two VLANs to delete and both required different solutions. I had created VLANs on both eth1 and eth2.
For eth2's VLAN, in the settings (gear icon on the VLAN) I set both IPv4 and IPv6 to DISABLED, and then I was able to delete the VLAN!
For eht1's VLAN, trying to disable the VLAN wouldn't work. It never disabled. I had to disable the main LAN's IPv4 and IPv6 first, and thenI could disable them on the VLAN, and then I could delete the VLAN, and then of course turn IPv4 and 6 back on in the LAN.
