There's a limitation in WAN/LAN routing for ER-X-SFP (link), keep it in mind. Don't expect speeds > 500Mbps.
I needed to use ethtool to fix autoneg not working:
ethtool -s eth5 speed 1000 autoneg off duplex full
Concerning your VDSL, idk.
I have moved to BPI-R3 meanwhile.