I dont have one but am curious.
For my vlan ISP connection I just define vlanXX attached to etherportY.
I ensure both the vlan and etherport are part of the WAN interface list.
The vlan is the entity that is associated with DHCP client.
For PPPOE
it looks more like this
/interface ethernet
set [ find default-name=ether5 ] name=e5-wan
/interface pppoe-client
add add-default-route=yes disabled=no interface=e5-wan name=pppoe-wan user=something@anything.com
What do you do when one has to associate with a VLAN??????
/interface ethernet
set [ find default-name=ether5 ] name=e5-wan
/interface vlan
add interface=e5-wan name=vlanWAN vlan-id=xx
/interface pppoe-client
add add-default-route=yes disabled=no interface=vlanWAN name=pppoe-wan user=something@anything.com