(See also Setting up Fydelia HotSpot on your MikroTik Routerboard)
To create a HotSpot on a VLAN, follow these steps:
From the left menu click BRIDGE
Add a new bridge. We'll call this "bridge-hotspot":
Click Apply and OK
Click the PORTS tab
Add a new port:
Assign your desired VLAN (vlan 10 in this case) to the bridge you created in the previous step
Click Apply and OK
This means that, during your HotSpot setup, you can select "bridge-hotspot" as the interface: