Any default routes which are learned via router advertisements cannot be redistributed with BGP VPN. If you look at the BGP VPN redistribution settings, the closest match for the route type is DHCP, but RA default routes are not marked as such in the routing table - you can see they are marked with “g”. Therefore these routes are not able to be exported.
There should be another option in the BGP VPN redistribute settings to match routes learned via router advertisements.
This is on ROS 7.21.3