Unluckily, I’ve no PPPoE modem to test this configuration at home. Let’s try again together! 
This is my networks DB:
db networks show
eth0=ethernet
bootproto=none
device=eth0
gateway=192.168.122.1
hwaddr=52:54:00:F5:C0:99
ipaddr=192.168.122.94
netmask=255.255.255.0
onboot=yes
role=green
eth1=ethernet
hwaddr=52:54:00:af:4d:35
role=
eth1.500=vlan
role=pppoe
eth2=ethernet
hwaddr=52:54:00:25:75:79
role=
ppp0=xdsl
AuthType=auto
Password=test
name=PPPoE
provider=xDSL provider
role=red
user=test
To configure the VLAN the commands should be:
db networks set eth1.500 vlan role pppoe
db networks settype ppp0 xdsl
db networks setprop ppp0 role red user test Password test
signal-event interface-update
Please attach relevant lines from /var/log/messages. For instance
Sep 21 13:28:16 vm8 esmith::event[9663]: Bringing up interface eth1.500: #033[60G[#033[0;32m OK #033[0;39m]#015
Sep 21 13:28:16 vm8 pppd[10423]: pppd 2.4.5 started by root, uid 0
Sep 21 13:28:16 vm8 pppd[10423]: Using interface ppp0
Sep 21 13:28:16 vm8 pppd[10423]: Connect: ppp0 <–> /dev/pts/1
…
Sep 21 13:28:47 vm8 pppd[10423]: Connection terminated.
Sep 21 13:28:47 vm8 pppd[10423]: Modem hangup
Sep 21 13:28:51 vm8 pppoe[10424]: Timeout waiting for PADO packets
Sep 21 13:28:51 vm8 pppd[10423]: Exit.