The error is:
Jun 3 08:40:21 vyos-test-1 pppd[9262]: In file /etc/ppp/peers/pppoe1: unrecognized option 'ifname'
Simply removing the offending option and leaving only ipparam appears to have no ill effects, PPPoE starts as intended:
vyos@vyos-test-1# run show interfaces pppoe
pppoe1: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1492 qdisc pfifo_fast state UNKNOWN group default qlen 3
link/ppp
inet 10.44.0.100 peer 10.44.0.1/32 scope global pppoe1
valid_lft forever preferred_lft forever
RX: bytes packets errors dropped overrun mcast
1138 17 0 0 0 0
TX: bytes packets errors dropped carrier collsns
1144 17 0 0 0 0
vyos@vyos-test-1# cat /var/run/pppoe1.pid
12992