On VyOS 1.2-rolling-201910180117 PPPoE clients fail to start on system boot. (In my case PPPoE is configured over an Ethernet VLAN.) (This setup worked perfectly on VyOS 1.1.8.)
Looking through journalctl -b 0 (i.e. last boot logs), I see the following:
pppd: In file /etc/ppp/peers/pppoe0: unrecognized option 'eth0.2'
Perhaps the VLAN interface is created only moments after PPPoE starts.
Also related to this. If for any reason the pppd process is killed, the PPPoE interfaces are not brought up again.