Configuring a DHCPv6 server on a specific VLAN on a MikroTik device to automatically assign IPv6 addresses to devices in that VLAN involves several steps.

We guide you through the general process using RouterOS, MikroTik's operating system:

Step 1: Create the VLAN

1. Access your MikroTik device using WinBox or the web interface.
2. Go to “Interfaces” and then to “+”, to create a new VLAN.
3. Configure the VLAN assigning it a name, specifying the “Parent” interface on which it will operate and defining its “VLAN ID”.

Step 2: Assign the VLAN to a Bridge Interface (if necessary)

If you don't already have a bridge created, you will need to create one and then add your VLAN and the necessary interfaces to the bridge:

1. Go to “Bridge”, then to “Bridge” again, and create a new bridge if necessary.
2. Add your VLAN and any relevant interfaces to the bridge going to “Ports” within the “Bridge” menu and adding each one as a bridge port.

Step 3: Configure the IPv6 Address for the VLAN

1. Go to “IPv6”, then to “Addresses”.
2. Add a new IPv6 address for your VLAN. You can specify the address manually or use a delegated prefix if your ISP provides one. Make sure you select the interface of your VLAN or bridge (if the VLAN is on a bridge).

Step 4: Configure the DHCPv6 Server

1. Go to “IPv6”, then to “DHCP Server” and select “DHCP”.
2. Create a new DHCPv6 server by clicking the “+” button.
3. Select VLAN interface for which you are configuring the DHCPv6 server.
4. Specifies the IPv6 address pool that the DHCPv6 server will use to assign to clients. If you have not created a pool yet, you will need to create one in “IPv6” > “Pool”.
5. Set additional options as necessary, such as gateway (if applicable), lease time, and DNS servers.

Step 5: Configure the IPv6 Address Pool

1. Go to “IPv6”, then to “Pool”.
2. Create a new pool specifying the range of IPv6 addresses that will be available to devices in your VLAN.

Step 6: Secure the IPv6 Route

Make sure you have an appropriate route configured for IPv6 traffic to allow devices in your VLAN to access the broader network or Internet:

1. Go to “IPv6”, then to “Routes”.
2. Verify that you have a default route configured and, if necessary, configure a route that points to your ISP's gateway or another router that handles IPv6 traffic.

This process will allow you to configure a DHCPv6 server on a specific VLAN on your MikroTik device, providing automatic IPv6 addresses to devices connected to that VLAN.

Remember that the exact steps may vary slightly depending on the version of RouterOS and your specific network configuration.