OpenVPN may do it.
Discaimer: I've NEVER architected a network with a need for a site-to-site VPN bridge. Site-to-site bridge, sure, using L2 switches. VPN, absolutely, but they have always been routed.
I had initially started to write an "It can't be done" reply, but started reading openVPN's doc's and howtos. OpenVPN supports a bridged VPN config, but I'm not clear if it works with site-to-site, or only in a road warrior scenario..
Regardless, a site-to-site bridged VPN would be a huge waste of network and processor bandwidth.