Hi,
Could you please try the following:
Navigate to Network -> Interfaces -> edit your VLAN interface -> Physical settings -> Turn on 'bridge interfaces' (Make sure physical interface eth0.200 is assigned)-> Save & Apply -> Save & Apply on the interfaces page. Then restart the router.
Also, check you firewall zone settings as these should allow traffic from your VLAN zone.
Have tested this configuration and it works.
If this does not help, please, replicate the issue and download a troubleshoot file from System -> Administration -> Troubleshoot. Attach the troubleshoot file to your question by editing it.
Kind Regards,
Andzej