You don't make it clear wether you are using BRI or PRI for this but the problem you may be having is that VPNs must have at least one end of the link supported by a permanent internet connection. ISDN to ISDN won't work unless both connections happen to be live at the same time. If one end is permanent, then the temporary end must initiate the communication. Thus isdn is ideal for RAS VPNs, but unusable for site to site. To make low cost VPN solutions, I recommend trying to find a cable provider who can give you permanent access, or ideally pay for a small leased line at each end.
We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...