Is it even possible to assign VLAN to each SSID on OpenWrt?

Installing and Using OpenWrt Network and Wireless Configuration
2

Yes

(Generic instructions follow)

  • Create a bridge interface for each VLAN you want to associate with one or more SSIDs, call it "vlan100" for clarity (this does not set it to be a VLAN, it's just a name)
  • "Bridge over" the sub-interface of the physical interface connected to the bridge, eth0.100 for example
  • Configure the wireless interface to attach to the bridge you just created
  • Adjust your switch, as needed. Note that many switches have a limit of 16 or 128 table entries, so larget VLAN tags will require use of the vid and pvid options in the config.

(If that's not obvious on how to accomplish, please let us know if you prefer to configure with files, or with LuCI)

As an example of file-based config of the bridge interface:

config interface 'vlan100'
	option type 'bridge'
	option stp '1'
	option ifname 'eth0.100'
	option proto 'none'
	option auto '1'
	option delegate '0'

It is option ifname 'eth0.100' that sets the VLAN tag, not the interface name.