Changing the base template controlling dhcp6c config generation seems to do the trick.
Folowing basic pppoe example provided on Vyos main website :
// config
interfaces {
...
pppoe pppoe0 {
...
dhcpv6-options {
pd 0 {
interface eth0 {
address "1"
sla-id "0"
}
length "56"
}
rapid-commit
}
ipv6 {
address {
autoconf
}
}
}
}