The Asus AC66U should work fine with VLANs and is reportedly working directly connected to the ONT in other countries. Also the Original firmware seems to be very nice and allow you console access to the embedded linux system and even install software inside it.
To setup VLAN connection, Select the WAN menu, under the "Special requirements from ISP" section select "Manual" and in the "Internet" subsection, put the VLAN ID in the VID field, it should do the trick. PS: As you can see you can forward IP telephony net to physical port 3 :-).
Hope it helps.