In my main-router i use the old /etc/network/interfaces to create vlans. My next system uses systemd. There is some documentstion in my wiki about it and afair i added some basic configs in my router-images repo
https://wiki.fw-web.de/doku.php?id=en:linux:systemd#vlan
Seems i have not added vlan files to images repo,but the pppoe service
but you need the ppp config file too where you point to the vlan interface. I created the ppp file (named provider in systemd service) using pppoeconf in debian
in my system the ppp-file (/etc/ppp/peers/bambit-internet) looks like this (not using systemd service here,so infinite tries):
# Minimalistic default options file for DSL/PPPoE connections
noipdefault
defaultroute
replacedefaultroute
hide-password
#lcp-echo-interval 30
#lcp-echo-failure 4
#updetach
noauth
persist
#mtu 1492
#Terminate after n consecutive failed connection attempts. A value of 0 means no limit. The default value is 10.
maxfail 0
#Specifies how many seconds to wait before re-initiating the link after it terminates. This option only has any effect if the persist or demand option is used. The holdoff period is not applied if the link was terminated because it was idle.
holdoff 60
plugin rp-pppoe.so wan.140
user "username"
#usepeerdns
unit 8
there is also /etc/ppp/chap-secrets and /etc/ppp/pap-secrets where you have to put your username/password for your provider. I have credentials in both files but chap (the better method) is used. If this is not the case in your test you can try require-chap in the peers file