So it turns out that if you try to convince Vista’s Network Diagnostics to tell you why it wasn’t able to establish an IPSEC VPN connection, and it tells you that either “There is no route available between the two computers” -

Or that “Network diagnostics pinged the remote host but did not receive a response” -

What it might really be trying to tell you is that the preshared key that you set in the VPN properties has somehow been forgotten.

It seems so obvious now.